Acute appendicitis is typically managed by surgery. However, in uncomplicated cases, antibiotics are effective and safe. While antibiotics are effective for treating uncomplicated appendicitis, 26% of people had a recurrence within a year and required eventual appendectomy. They work less well if an appendicolith is present. Cost effectiveness of surgery versus antibiotics is unclear.

Pain medications (such as morphine) do not appear to affect the accuracy of the clinical diagnosis of appendicitis and therefore should be given early in the patient's care. Historically there were concerns among some general surgeons that analgesics would affect the clinical exam in children, and some recommended that they not be given until the surgeon was able to examine the person.

Depending on the severity of the patient's state, the management of peritonitis may include:

- General supportive measures such as vigorous intravenous rehydration and correction of electrolyte disturbances.

- Antibiotics are usually administered intravenously, but they may also be infused directly into the peritoneum. The empiric choice of broad-spectrum antibiotics often consist of multiple drugs, and should be targeted against the most likely agents, depending on the cause of peritonitis (see above); once one or more agents are actually isolated, therapy will of course be target on them.

- Gram positive and gram negative organisms must be covered. Out of the cephalosporins, cefoxitin and cefotetan can be used to cover gram positive bacteria, gram negative bacteria, and anaerobic bacteria. Beta-lactams with beta lactamase inhibitors can also be used, examples include ampicillin/sulbactam, piperacillin/tazobactam, and ticarcillin/clavulanate. Carbapenems are also an option when treating primary peritonitis as all of the carbapenems cover gram positives, gram negatives, and anaerobes except for ertapenem. The only fluoroquinolone that can be used is moxifloxacin because this is the only fluoroquinolone that covers anaerobes. Finally, tigecycline is a tetracycline that can be used due to its coverage of gram positives and gram negatives. Empiric therapy will often require multiple drugs from different classes.

- Surgery (laparotomy) is needed to perform a full exploration and lavage of the peritoneum, as well as to correct any gross anatomical damage that may have caused peritonitis. The exception is spontaneous bacterial peritonitis, which does not always benefit from surgery and may be treated with antibiotics in the first instance.

Supportive measures may be instituted prior to surgery. These measures include fluid resuscitation. Intravenous opioids can be used for pain control.

Antibiotics are often not needed. If used they should target enteric organisms (e.g. Enterobacteriaceae), such as "E. coli" and "Bacteroides". This may consist of a broad spectrum antibiotic; such as piperacillin-tazobactam, ampicillin-sulbactam, ticarcillin-clavulanate (Timentin), a third generation cephalosporin (e.g.ceftriaxone) or a quinolone antibiotic (such as ciprofloxacin) and anaerobic bacteria coverage, such as metronidazole. For penicillin allergic people, aztreonam or a quinolone with metronidazole may be used.

In cases of severe inflammation, shock, or if the person has higher risk for general anesthesia (required for cholecystectomy), an interventional radiologist may insert a percutaneous drainage catheter into the gallbladder ('percutaneous cholecystostomy tube') and treat the person with antibiotics until the acute inflammation resolves. A cholecystectomy may then be warranted if the person's condition improves.

Homeopathic approaches to treating cholecystitis have not been validated by evidence and should not be used in place of surgery.

For most people with acute cholecystitis, the treatment of choice is surgical removal of the gallbladder, laparoscopic cholecystectomy. Laparoscopic cholecystectomy is performed using several small incisions located at various points across the abdomen. Several studies have demonstrated the superiority of laparoscopic cholecystectomy when compared to open cholecystectomy (using a large incision in the right upper abdomen under the rib cage). People undergoing laparoscopic surgery report less incisional pain postoperatively as well as having fewer long term complications and less disability following the surgery. Additionally, laparoscopic surgery is associated with a lower rate of surgical site infection.

During the days prior to laparoscopic surgery, studies showed that outcomes were better following early removal of the gallbladder, preferably within the first week. Early laparoscopic cholecystectomy (within 7 days of visiting a doctor with symptoms) as compared to delayed treatment (more than 6 weeks) may result in shorter hospital stays and a decreased risk of requiring an emergency procedure. There is no difference in terms of negative outcomes including bile duct injury or conversion to open cholecystectomy. For early cholecystectomy, the most common reason for conversion to open surgery is inflammation that hides Calot's triangle. For delayed surgery, the most common reason was fibrotic adhesions.

Surgical intervention is nearly always required in form of exploratory laparotomy and closure of perforation with peritoneal wash. Occasionally they may be managed laparoscopically.

Conservative treatment including intravenous fluids, antibiotics, nasogastric aspiration and bowel rest is indicated only if the person is nontoxic and clinically stable.

Initial management includes the relief of symptoms and correcting electrolyte and fluid imbalance that may occur with vomiting. Antiemetics, such as dimenhydrinate, are used to treat the nausea. Pain may be treated with anti-inflammatories, NSAIDs such as ketorolac or diclofenac. Opioids, such as morphine, less commonly may be used. NSAIDs are more or less equivalent to opioids. Hyoscine butylbromide, an antispasmodic, is also indicated in biliary colic.

In biliary colic, the risk of infection is minimal and therefore antibiotics are not required. Presence of infection indicates cholecystitis.

It is unclear whether those experiencing a gallstone attack should receive surgical treatment or not. The scientific basis to assess whether surgery outperformed other treatment was insufficient and better studies were needed as of a SBU report in 2017. Treatment of biliary colic is dictated by the underlying cause. The presence of gallstones, usually visualized by ultrasound, generally necessitates a surgical treatment (removal of the gall bladder, typically via laparoscopy). Removal of the gallbladder with surgery, known as a cholecystectomy, is the definitive surgical treatment for biliary colic. A 2013 Cochrane review found tentative evidence to suggest that early gallbladder removal may be better than delayed removal. Early laparoscopic cholescystectomy happens within 72 hours of diagnosis. In a Cochrane review that evaluated receiving early versus delayed surgery, they found that 23% of those who waited on average 4 months ended up in hospital for complications, compared to none with early intervention with surgery. Early intervention has other advantages including reduced number of visits to the emergency department, less conversions to an open surgery, less operating time required, reduced time in hospital post operatively. The Swedish agency SBU estimated in 2017 that increasing acute phase surgeries could

free multiple in-hospital days per patient and would additionally spare pain and suffering in wait of receiving an operation. The report found that those with acute inflammation of the gallbladder can be surgically treated in the acute phase, within a few days of symptom debut, without increasing the risk for complications (compared to when the surgery is done later in an asymptomatic stage).

If properly treated, typical cases of surgically correctable peritonitis (e.g., perforated peptic ulcer, appendicitis, and diverticulitis) have a mortality rate of about <10% in otherwise healthy patients. The mortality rate rises to about 40% in the elderly, or in those with significant underlying illness, as well as cases that present late (after 48 hours).

Without being treated, generalised peritonitis almost always causes death. The stage magician Harry Houdini died this way, having contracted streptococcus peritonitis after his appendix ruptured and was removed too late to prevent spread of the infection.

In the management of small bowel obstructions, a commonly quoted surgical aphorism is: "never let the sun rise or set on small-bowel obstruction" because about 5.5% of small bowel obstructions are ultimately fatal if treatment is delayed. However improvements in radiological imaging of small bowel obstructions allow for confident distinction between simple obstructions, that can be treated conservatively, and obstructions that are surgical emergencies (volvulus, closed-loop obstructions, ischemic bowel, incarcerated hernias, etc.).

A small flexible tube (nasogastric tube) may be inserted through the nose into the stomach to help decompress the dilated bowel. This tube is uncomfortable but does relieve the abdominal cramps, distention, and vomiting. Intravenous therapy is utilized and the urine output is monitored with a catheter in the bladder.

Most people with SBO are initially managed conservatively because in many cases, the bowel will open up. Some adhesions loosen up and the obstruction resolves. However, when conservative management is undertaken, the patient is examined several times a day, and X-ray images are obtained to ensure that the individual is not getting clinically worse.

Conservative treatment involves insertion of a nasogastric tube, correction of dehydration and electrolyte abnormalities. Opioid pain relievers may be used for patients with severe pain. Antiemetics may be administered if the patient is vomiting. Adhesive obstructions often settle without surgery. If the obstruction is complete a surgery is usually required.

Most patients do improve with conservative care in 2–5 days. However, on some occasions, the cause of obstruction may be a cancer and in such cases, surgery is the only treatment. These individuals undergo surgery where the cause of SBO is removed. Individuals who have bowel resection or lysis of adhesions usually stay in the hospital a few more days until they are able to eat and walk.

Small bowel obstruction caused by Crohn's disease, peritoneal carcinomatosis, sclerosing peritonitis, radiation enteritis, and postpartum bowel obstruction are typically treated conservatively, i.e. without surgery.

Antiemetic medications may be helpful for treating vomiting in children. Ondansetron has some utility, with a single dose being associated with less need for intravenous fluids, fewer hospitalizations, and decreased vomiting. Metoclopramide might also be helpful. However, the use of ondansetron might possibly be linked to an increased rate of return to hospital in children. The intravenous preparation of ondansetron may be given orally if clinical judgment warrants. Dimenhydrinate, while reducing vomiting, does not appear to have a significant clinical benefit.

Some causes of bowel obstruction may resolve spontaneously; many require operative treatment. In adults, frequently the surgical intervention and the treatment of the causative lesion are required. In malignant large bowel obstruction, endoscopically placed self-expanding metal stents may be used to temporarily relieve the obstruction as a bridge to surgery, or as palliation. Diagnosis of the type of bowel obstruction is normally conducted through initial plain radiograph of the abdomen, luminal contrast studies, computed tomography scan, or ultrasonography prior to determining the best type of treatment.

Antibiotics are not usually used for gastroenteritis, although they are sometimes recommended if symptoms are particularly severe or if a susceptible bacterial cause is isolated or suspected. If antibiotics are to be employed, a macrolide (such as azithromycin) is preferred over a fluoroquinolone due to higher rates of resistance to the latter. Pseudomembranous colitis, usually caused by antibiotic use, is managed by discontinuing the causative agent and treating it with either metronidazole or vancomycin. Bacteria and protozoans that are amenable to treatment include "Shigella" "Salmonella typhi", and "Giardia" species. In those with "Giardia" species or "Entamoeba histolytica", tinidazole treatment is recommended and superior to metronidazole. The World Health Organization (WHO) recommends the use of antibiotics in young children who have both bloody diarrhea and fever.

Distal or sigmoid, fecalomas can often be disimpacted digitally or by a catheter which carries a flow of disimpaction fluid (water or other solvent or lubricant). Surgical intervention in the form of sigmoid colectomy or proctocolectomy and ileostomy may be required only when all conservative measures of evacuation fail.

No clear beneficial effect from spinal manipulation or massage has been shown. Further, as there is no evidence of safety for cervical manipulation for baby colic, it is not advised. There is a case of a three-month-old dying following manipulation of the neck area.

No evidence supports the efficacy of so-called "gripe water", and its use poses risks, especially in formulations that include alcohol or sugar. Evidence does not support lactase, or supplementing formula with probiotics. The use of the probiotic "Lactobacillus reuteri" in babies who are breastfed has tentative evidence.

Dietary changes by infants are generally not needed. In mothers who are breastfeeding, a hypoallergenic diet by the mother — not eating milk and dairy products, eggs, wheat, and nuts — may improve matters, while elimination of only cow’s milk does not seem to produce any improvement. In formula-fed infants, switching to a soy-based or hydrolyzed protein formula may help. Evidence of benefit is greater for hydrolyzed protein formula with the benefit from soy based formula being disputed. Additionally both these formulas have greater cost and are not as palatable. Supplementation with fiber has no benefit.

Differentiation of DIOS from constipation is generally performed by unit specializing in the treatment of cystic fibrosis. Adequate hydration and an aggressive regimen of laxatives are essential for treatment and prevention of DIOS. Osmotic laxatives such as polyethylene glycol are preferred. Individuals prone to DIOS tend to be at risk for repeated episodes and often require maintenance therapy with pancreatic enzyme replacement, hydration and laxatives (if the symptoms are also mild).

Oral contrast instillation into the colon/ileum under radiological control has been found to reduce the need for surgical intervention.

Corticosteroids are the mainstay of therapy with a 90% response rate in some studies. Appropriate duration of steroid treatment is unknown and relapse often necessitates long term treatment. Various steroid sparing agents e.g. sodium cromoglycate (a stabilizer of mast cell membranes), ketotifen (an antihistamine), and montelukast (a selective, competitive leukotriene receptor antagonist) have been proposed, centering on an allergic hypothesis, with mixed results. An elimination diet may be successful if a limited number of food allergies are identified.

The differential diagnoses of acute abdomen include but are not limited to:

1. Acute appendicitis

2. Acute peptic ulcer and its complications

3. Acute cholecystitis

4. Acute pancreatitis

5. Acute intestinal ischemia (see section below)

6. Acute diverticulitis

7. Ectopic pregnancy with tubal rupture

8. Ovarian torsion

9. Acute peritonitis (including hollow viscus perforation)

10. Acute ureteric colic

11. Bowel volvulus

12. Bowel obstruction

13. Acute pyelonephritis

14. Adrenal crisis

15. Biliary colic

16. Abdominal aortic aneurysm

17. Familial Mediterranean fever

18. Hemoperitoneum

19. Ruptured spleen

20. Kidney stone

21. Sickle cell anaemia

Treatment is often started without confirmation of infection because of the serious complications that may result from delayed treatment. Treatment depends on the infectious agent and generally involves the use of antibiotic therapy. If there is no improvement within two to three days, the patient is typically advised to seek further medical attention. Hospitalization sometimes becomes necessary if there are other complications. Treating sexual partners for possible STIs can help in treatment and prevention.

For women with PID of mild to moderate severity, parenteral and oral therapies appear to be effective. It does not matter to their short- or long-term outcome whether antibiotics are administered to them as inpatients or outpatients. Typical regimens include cefoxitin or cefotetan plus doxycycline, and clindamycin plus gentamicin. An alternative parenteral regimen is ampicillin/sulbactam plus doxycycline. Erythromycin-based medications can also be used. Another alternative is to use a parenteral regimen with ceftriaxone or cefoxitin plus doxycycline. Clinical experience guides decisions regarding transition from parenteral to oral therapy, which usually can be initiated within 24–48 hours of clinical improvement.

Gastrointestinal perforation, also known as ruptured bowel, is a hole in the wall of part of the gastrointestinal tract. The gastrointestinal tract includes the esophagus, stomach, small intestine, and large intestine. Symptoms include severe abdominal pain and tenderness. When the hole is in the stomach or early part of the small intestine the onset of pain is typically sudden while with a hole in the large intestine onset may be more gradual. The pain is usually constant in nature. Sepsis, with an increased heart rate, increased breathing rate, fever, and confusion may occur.

The cause can include trauma such as from a knife wound, eating a sharp object, or a medical procedure such as colonoscopy, bowel obstruction such as from a volvulus, colon cancer, or diverticulitis, stomach ulcers, ischemic bowel, and a number of infections including "C. difficile". A hole allows intestinal contents to enter the abdominal cavity. The entry of bacteria results in a condition known as peritonitis or in the formation of an abscess. A hole in the stomach can also lead to a chemical peritonitis due to gastric acid. A CT scan is typically the preferred method of diagnosis; however, free air from a perforation can often be seen on plain X-ray.

Perforation anywhere along the gastrointestinal tract typically requires emergency surgery in the form of an exploratory laparotomy. This is usually carried out along with intravenous fluids and antibiotics. A number of different antibiotics may be used such as piperacillin/tazobactam or the combination of ciprofloxacin and metronidazole. Occasionally the hole can be sewn closed while other times a bowel resection is required. Even with maximum treatment the risk of death can be as high as 50%. A hole from a stomach ulcer occurs in about 1 per 10,000 people per year, while one from diverticulitis occurs in about 0.4 per 10,000 people per year.

Stable patients presenting to A&E (accident and emergency department) or ER (emergency room) with severe abdominal pain will almost always have an abdominal x-ray and/or a CT scan. These tests can provide a differential diagnosis between simple and complex pathologies. However, in the unstable patient, fluid resuscitation and a FAST-ultrasound are done first, and if the latter is positive for free fluid, straight to surgery. They may also provide evidence to the doctor whether surgical intervention is necessary.

Patients will also most likely receive a complete blood count (or full blood count in the U.K.), looking for characteristic findings such as neutrophilia in appendicitis.

Traditionally, the use of opiates or other painkillers in patients with an acute abdomen has been discouraged before the clinical examination, because these would alter the examination. However, the scientific literature does not reveal any negative results from these alterations.

Antibiotics are the first line of treatment in acute prostatitis. Antibiotics usually resolve acute prostatitis infections in a very short time, however a minimum of two to four weeks of therapy is recommended to eradicate the offending organism completely. Appropriate antibiotics should be used, based on the microbe causing the infection. Some antibiotics have very poor penetration of the prostatic capsule, others, such as ciprofloxacin, trimethoprim/sulfamethoxazole, and tetracyclines such as doxycycline penetrate prostatic tissue well. In acute prostatitis, penetration of the prostate is not as important as for category II because the intense inflammation disrupts the prostate-blood barrier. It is more important to choose a bactericidal antibiotic (kills bacteria, e.g., a fluoroquinolone antibiotic) rather than a bacteriostatic antibiotic (slows bacterial growth, e.g. tetracycline) for acute potentially life-threatening infections.

Severely ill patients may need hospitalization, while nontoxic patients can be treated at home with bed rest, analgesics, stool softeners, and hydration. Men with acute prostatitis complicated by urinary retention are best managed with a suprapubic catheter or intermittent catheterization. Lack of clinical response to antibiotics should raise the suspicion of an abscess and prompt an imaging study such as a transrectal ultrasound (TRUS).

Treatment is surgical, potentially with a laparoscopic resection. In patients with bleeding, strangulation of bowel, bowel perforation or bowel obstruction, treatment involves surgical resection of both the Meckel's diverticulum itself along with the adjacent bowel segment, and this procedure is called a "small bowel resection". In patients without any of the aforementioned complications, treatment involves surgical resection of the Meckel's diverticulum only, and this procedure is called a simple diverticulectomy.

With regards to asymptomatic Meckel's diverticulum, some recommend that a search for Meckel's diverticulum should be conducted in every case of appendectomy/laparotomy done for acute abdomen, and if found, Meckel's diverticulectomy or resection should be performed to avoid secondary complications arising from it.

Patients with uncomplicated acute pericarditis can generally be treated and followed up in an outpatient clinic. However, those with high risk factors for developing complications (see above) will need to be admitted to an inpatient service, most likely an ICU setting. High risk patients include the following:

- subacute onset

- high fever (> 100.4 F/38 C) and leukocytosis

- development of cardiac tamponade

- large pericardial effusion (echo-free space > 20 mm) resistant to NSAID treatment

- immunocompromised

- history of oral anticoagulation therapy

- acute trauma

- failure to respond to seven days of NSAID treatment

Pericardiocentesis is a procedure whereby the fluid in a pericardial effusion is removed through a needle. It is performed under the following conditions:

- presence of moderate or severe cardiac tamponade

- diagnostic purpose for suspected purulent, tuberculosis, or neoplastic pericarditis

- persistent symptomatic pericardial effusion

NSAIDs in "viral" or "idiopathic" pericarditis. In patients with underlying causes other than viral, the specific etiology should be treated. With idiopathic or viral pericarditis, NSAID is the mainstay treatment. Goal of therapy is to reduce pain and inflammation. The course of the disease may not be affected. The preferred NSAID is ibuprofen because of rare side effects, better effect on coronary flow, and larger dose range. Depending on severity, dosing is between 300–800 mg every 6–8 hours for days or weeks as needed. An alternative protocol is aspirin 800 mg every 6–8 hours. Dose tapering of NSAIDs may be needed. In pericarditis following acute myocardial infarction, NSAIDs other than aspirin should be avoided since they can impair scar formation. As with all NSAID use, GI protection should be engaged. Failure to respond to NSAIDs within one week (indicated by persistence of fever, worsening of condition, new pericardial effusion, or continuing chest pain) likely indicates that a cause other than viral or idiopathic is in process.

Colchicine, which has been essential to treat recurrent pericarditis, has been supported for routine use in acute pericarditis by recent prospective studies. Colchicine can be given 0.6 mg twice a day (0.6 mg daily for patients <70 kg) for 3 months following an acute attack. It should be considered in all patients with acute pericarditis, preferably in combination with a short-course of NSAIDs. For patients with a first episode of acute idiopathic or viral pericarditis, they should be treated with an NSAID plus colchicine 1–2 mg on first day followed by 0.5 daily or twice daily for three months. It should be avoided or used with caution in patients with severe renal insufficiency, hepatobiliary dysfunction, blood dyscrasias, and gastrointestinal motility disorders.

Corticosteroids are usually used in those cases that are clearly refractory to NSAIDs and colchicine and a specific cause has not been found. Systemic corticosteroids are usually reserved for those with autoimmune disease.