Among US adults older than 55, 4% are taking medication and or supplements that put them at risk of a major drug interaction. Potential drug-drug interactions have increased over time and are more common in the low educated elderly even after controlling for age, sex, place of residence, and comorbidity.

A drug interaction is a situation in which a substance (usually another drug) affects the activity of a drug when both are administered together. This action can be synergistic (when the drug's effect is increased) or antagonistic (when the drug's effect is decreased) or a new effect can be produced that neither produces on its own. Typically, interactions between drugs come to mind (drug-drug interaction). However, interactions may also exist between drugs and foods (drug-food interactions), as well as drugs and medicinal plants or herbs (drug-plant interactions). People taking antidepressant drugs such as monoamine oxidase inhibitors should not take food containing tyramine as hypertensive crisis may occur (an example of a drug-food interaction). These interactions may occur out of accidental misuse or due to lack of knowledge about the active ingredients involved in the relevant substances.

It is therefore easy to see the importance of these pharmacological interactions in the practice of medicine. If a patient is taking two drugs and one of them increases the effect of the other it is possible that an overdose may occur. The interaction of the two drugs may also increase the risk that side effects will occur. On the other hand, if the action of a drug is reduced it may cease to have any therapeutic use because of under dosage. Notwithstanding the above, on occasion these interactions may be sought in order to obtain an improved therapeutic effect. Examples of this include the use of codeine with paracetamol to increase its analgesic effect. Or the combination of clavulanic acid with amoxicillin in order to overcome bacterial resistance to the antibiotic. It should also be remembered that there are interactions that, from a theoretical standpoint, may occur but in clinical practice have no important repercussions.

The pharmaceutical interactions that are of special interest to the practice of medicine are primarily those that have negative effects for an organism. The risk that a pharmacological interaction will appear increases as a function of the number of drugs administered to a patient at the same time. Over a third (36%) of older adults in the U.S. regularly use 5 or more medications or supplements and 15% are potentially at risk for a major drug-drug interaction. Both the use of medications and subsequent adverse drug interactions have increased significantly between 2005-2011.

It is possible that an interaction will occur between a drug and another substance present in the organism (i.e. foods or alcohol). Or in certain specific situations a drug may even react with itself, such as occurs with dehydration. In other situations, the interaction does not involve any effect on the drug. In certain cases, the presence of a drug in an individual's blood may affect certain types of laboratory analysis (analytical interference).

It is also possible for interactions to occur outside an organism before administration of the drugs has taken place. This can occur when two drugs are mixed, for example, in a saline solution prior to intravenous injection. Some classic examples of this type of interaction include that thiopentone and suxamethonium should not be placed in the same syringe and same is true for benzylpenicillin and heparin. These situations will all be discussed under the same heading due to their conceptual similarity.

Drug interactions may be the result of various processes. These processes may include alterations in the pharmacokinetics of the drug, such as alterations in the absorption, distribution, metabolism, and excretion (ADME) of a drug. Alternatively, drug interactions may be the result of the pharmacodynamic properties of the drug, e.g. the co-administration of a receptor antagonist and an agonist for the same receptor.

Common adverse drug reactions (≥ 1% of people) associated with use of the penicillins include diarrhoea, hypersensitivity, nausea, rash, neurotoxicity, urticaria, and superinfection (including candidiasis). Infrequent adverse effects (0.1–1% of people) include fever, vomiting, erythema, dermatitis, angioedema, seizures (especially in people with epilepsy), and pseudomembranous colitis.

Animals are often treated with antibiotics for infections they have developed. There are side effects of penicillin when it is used in animals. MRSA may develop in pets as a consequence of treatment.

Dogs may have side effects that include: pain in their joints, loss of appetite, vomiting, flatulence (instestinal gas), fungal infections and digestive problems. Like humans, dogs can have a similar side effect related to developing a serioys allergy. A serious and possibly fatal anaphylactic can occur. Side effects that are concurrent with anaphylaxis include: breathing problems and shock.

Cats and dogs have had adverse reactions to intravenous penicillin that include: hypothermia, pruritus, hypotension, tremors, seizures, blindness, vocalization, agitation, cardiac arrest and transient loss of vision.

Drug and alcohol use during pregnancy can lead to many health problems in the baby besides NAS. These may include:

- Birth defects

- Low birth weight

- Premature birth

- Small head circumference

- Sudden infant death syndrome (SIDS)

- Problems with development and behavior

Neonatal abstinence syndrome treatment can last from 1 week to 6 months. Even after medical treatment for NAS is over and babies leave the hospital, they may need continued treatment for weeks or months.

In addition to vaccine-specific factors, vets and owners should also consider pet-specific factors that have been shown to increase the risk of adverse reactions in both dogs and cats. Examples of such factors include:

- age,

- number of vaccinations per office visit,

- size,

- general health of the animal,

- breed,

- neutered status, and

- past vaccination history.

A 2012 study from the University of Michigan and the University of Pittsburgh published in the "Journal of the American Medical Association" analyzed information on 7.4 million discharges from 4,121 hospitals in 44 states, to measure trends and costs associated with NAS over the past decade. The study indicated that between 2000 and 2009, the number of mothers using opiates increased from 1.19 to 5.63 per 1,000 hospital births per year. Newborns with NAS were 19% more likely than all other hospital births to have low birthweight and 30% more like to have respiratory complications. Between 2000 and 2009, total hospital charges for NAS cases, adjusted for inflation, are estimated to have increased from $190 million to $720 million.

Neonatal abstinence syndrome in Canada are significant.

A little known and often misdiagnosed reaction to the rabies vaccine in dogs, this problem may develop near or over the vaccine administration site and around the vaccine material that was injected, or as a more widespread reaction. Symptoms include ulcers, scabs, darkening of the skin, lumps at the vaccine site, and scarring with loss of hair. In addition to the vaccination site, lesions most often develop on the ear flaps (pinnae), on the elbows and hocks, in the center of the footpads and on the face. Scarring may be permanent. Dogs do not usually seem ill, but may develop fever. Symptoms may show up within weeks of vaccination, or may take months to develop noticeably.

Dogs with active lesion development and / or widespread disease may be treated with pentoxyfylline, a drug that is useful in small vessel vasculitis, or tacrolimus, an ointment that will help suppress the inflammation in the affected areas.

Owners and veterinarians of dogs who have developed this type of reaction should review the vaccination protocol critically and try to reduce future vaccinations to the extent medically and legally possible. At the very least, vaccines from the same manufacturer should be avoided. It is also recommended that the location in which future vaccinations are administered should be changed to the rear leg, as far down on the leg as possible and should be given in the muscle rather than under the skin.

The life span in patients with Schnitzler syndrome has not been shown to differ much from the general population. Careful follow-up is advised, however. A significant proportion of patients develops a lymphoproliferative disorder as a complication, most commonly Waldenström's macroglobulinemia. This may lead to symptoms of hyperviscosity syndrome. AA amyloidosis has also been reported in people with Schnitzler syndrome.

Injection site reactions are allergic reactions that result in cutaneous necrosis that may occur at sites of medication injection, typically presenting in one of two forms, (1) those associated with intravenous infusion or (2) those related to intramuscular injection. Intra muscular injections may produce a syndrome called livedo dermatitis.

Collapsed veins are a common result of chronic use of intravenous injections. They are particularly common where injecting conditions are less than ideal, such as in the context of drug abuse.

Veins may become temporarily blocked if the internal lining of the vein swells in response to repeated injury or irritation. This may be caused by the needle, the substance injected, or donating plasma. Once the swelling subsides, the circulation will often become re-established.

Permanent vein collapse occurs as a consequence of:

- Long-term injecting

- Repeated injections, especially with blunt needles

- Poor technique

- Injection of substances which irritate the veins; in particular, injection of liquid methadone intended for oral use.

Smaller veins may collapse as a consequence of too much suction being used when pulling back against the plunger of the syringe to check that the needle is in the vein. This will pull the sides of the vein together and, especially if they are inflamed, they may stick together causing the vein to block. Removing the needle too quickly after injecting can have a similar effect.

Collapsed veins may never recover. Many smaller veins are created by the body to circulate the blood, but they are not adequate for injections or IVs.

Complications of benzodiazepine abuse include drug-related deaths due to overdose especially in combination with other depressant drugs such as opioids. Other complications include: blackouts and memory loss, paranoia, violence and criminal behaviour, risk-taking sexual behaviour, foetal and neonatal risks if taken in pregnancy, dependence, withdrawal seizures and psychosis. Injection of the drug carries risk of: thrombophlebitis, deep vein thrombosis, deep and superficial abscesses, pulmonary microembolism, rhabdomyolysis, tissue necrosis, gangrene requiring amputation, hepatitis B and C, as well as blood borne infections such as HIV infection (caused by sharing injecting equipment). Long-term use of benzodiazepines can worsen pre-existing depression and anxiety and may potentially also cause dementia with impairments in recent and remote memory functions.

Use is widespread among amphetamine users, with those that use amphetamines and benzodiazepines having greater levels of mental health problems and social deterioration. Benzodiazepine injectors are almost four times more likely to inject using a shared needle than non-benzodiazepine-using injectors. It has been concluded in various studies that benzodiazepine use causes greater levels of risk and psycho-social dysfunction among drug misusers.

Poly-drug users who also use benzodiazepines appear to engage in more frequent high-risk behaviors. Those who use stimulant and depressant drugs are more likely to report adverse reactions from stimulant use, more likely to be injecting stimulants and more likely to have been treated for a drug problem than those using stimulant but not depressant drugs.

More than 64,000 Americans died from drug overdoses in 2016. Since 2000, the US drug overdose death rate has gone from 6.2 per 100,000 persons in 2000 to 14.7 per 100,000 in 2014.

The National Center for Health Statistics report that 19,250 people died of accidental poisoning in the U.S. in the year 2004 (8 deaths per 100,000 population).

In 2008 testimony before a Senate subcommittee, Leonard J. Paulozzi, a medical epidemiologist at the Centers for Disease Control and Prevention stated that in 2005 more than 22,000 American lives were lost due to overdoses, and the number is growing rapidly. Paulozzi also testified that all available evidence suggests that unintentional overdose deaths are related to the increasing use of prescription drugs, especially opioid painkillers. However, the vast majority of overdoses are also attributable to alcohol. It is very rare for a victim of an overdose to have consumed just one drug. Most overdoses occur when drugs are ingested in combination with alcohol.

Drug overdose was the leading cause of injury death in 2013. Among people 25 to 64 years old, drug overdose caused more deaths than motor vehicle traffic crashes. There were 43,982 drug overdose deaths in the United States in 2013. Of these, 22,767 (51.8%) were related to prescription drugs.

The 22,767 deaths relating to prescription drug overdose in 2013, 16,235 (71.3%) involved opioid painkillers, and 6,973 (30.6%) involved benzodiazepines. Drug misuse and abuse caused about 2.5 million emergency department (ED) visits in 2011. Of these, more than 1.4 million ED visits were related to prescription drugs. Among those ED visits, 501,207 visits were related to anti-anxiety and insomnia medications, and 420,040 visits were related to opioid analgesics.

Benzodiazepines are a commonly abused class of drugs, although there is debate as to whether certain benzodiazepines have higher abuse potential than others. In animal and human studies the abuse potential of benzodiazepines is classed as moderate in comparison to other drugs of abuse.

Benzodiazepines are commonly abused by poly drug users, especially heroin addicts, alcoholics or amphetamine addicts when "coming down". but sometimes are misused in isolation as the primary drug of misuse. They can be misused to achieve the high that benzodiazepines produce or more commonly they are used to either enhance the effects of other CNS depressant drugs, to stave off withdrawal effects of other drugs or combat the effects of stimulants. As many as 30–50% of alcoholics are also benzodiazepine misusers. Drug abusers often abuse high doses which makes serious benzodiazepine withdrawal symptoms such as psychosis or convulsions more likely to occur during withdrawal.

Benzodiazepine abuse increases risk-taking behaviors such as unprotected sex and sharing of needles amongst intravenous abusers of benzodiazepines. Abuse is also associated with blackouts, memory loss, aggression, violence, and chaotic behavior associated with paranoia. There is little support for long-term maintenance of benzodiazepine abusers and thus a withdrawal regime is indicated when benzodiazepine abuse becomes a dependence. The main source of illicit benzodiazepines are diverted benzodiazepines obtained originally on prescription; other sources include thefts from pharmacies and pharmaceutical warehouses. Benzodiazepine abuse is steadily increasing and is now a major public health problem. Benzodiazepine abuse is mostly limited to individuals who abuse other drugs, i.e. poly-drug abusers. Most prescribed users do not abuse their medication, however, some high dose prescribed users do become involved with the illicit drug scene. Abuse of benzodiazepines occurs in a wide age range of people and includes teenagers and the old. The abuse potential or drug-liking effects appears to be dose related, with low doses of benzodiazepines having limited drug liking effects but higher doses increasing the abuse potential/drug-liking properties.

In immunology, the Arthus reaction (, ) is a type of local type III hypersensitivity reaction. Type III hypersensitivity reactions are immune complex-mediated, and involve the deposition of antigen/antibody complexes mainly in the vascular walls, serosa (pleura, pericardium, synovium), and glomeruli. This reaction is usually encountered in experimental settings following the injection of antigens.

Perhaps the most universal risk of sneezing is the spread of disease. Bacterial infections can spread to susceptible uninfected people via the spread of microscopic organisms suspended in the droplets expelled by a sneeze. Bacteria which commonly spread by sneezing include bacterial meningitis, strep throat, and tuberculosis. Viral infections can also be spread by sneezing. When a virus is expelled by a sneeze, its mucous membrane evaporates, and the virus becomes a droplet nucleus which can be inhaled by another person, thus spreading the virulent infection. Examples of virulent infections that spread by sneezing include measles, mumps, rubella, and influenza.

The psychological effects of occupational needlestick injuries can include health anxiety, anxiety about disclosure or transmission to a sexual partner, trauma-related emotions, and depression. These effects can cause self-destructive behavior or functional impairment in relationships and daily life. This is not mitigated by knowledge about disease transmission or PEP. Though some affected people have worsened anxiety during repeated testing, anxiety and other psychological effects typically abate after testing is complete. A minority of people affected by needlestick injuries may have lasting psychological effects, including post-traumatic stress disorder.

Treatment must also include a lumenal amoebicide to prevent reinvasion of tissues by amoebae still in the intestines (see Amoebiasis). After completion of treatment with tissue amebicides, administer luminal amebicides for eradication of the asymptomatic colonization state. Failure to use luminal agents can lead to relapse of infection in approximately 10% of patients.

While needlestick injuries have the potential to transmit bacteria, protozoa, viruses and prions, the risk of contracting hepatitis B, hepatitis C, and HIV is the highest. The World Health Organization estimated that in 2000, 66,000 hepatitis B, 16,000 hepatitis C, and 1,000 HIV infections were caused by needlestick injuries. In places with higher rates of blood-borne diseases in the general population, healthcare workers are more susceptible to contracting these diseases from a needlestick injury.

Hepatitis B carries the greatest risk of transmission, with 10% of exposed workers eventually showing seroconversion and 10% having symptoms. Higher rates of hepatitis B vaccination among the general public and healthcare workers have reduced the risk of transmission; non-healthcare workers still have a lower HBV vaccine rate and therefore a higher risk. The hepatitis C transmission rate has been reported at 1.8%, but newer, larger surveys have shown only a 0.5% transmission rate. The overall risk of HIV infection after percutaneous exposure to HIV-infected material in the health care setting is 0.3%. Individualized risk of blood-borne infection from a used biomedical sharp is further dependent upon additional factors. Injuries with a hollow-bore needle, deep penetration, visible blood on the needle, a needle located in a deep artery or vein, or a biomedical device contaminated with blood from a terminally ill patient increase the risk for contracting a blood-borne infection.

After a needlestick injury, certain procedures must be followed to minimize the risk of infection. Lab tests of the recipient should be obtained for baseline studies, including HIV, acute hepatitis panel (HAV IgM, HBsAg, HB core IgM, HCV) and for immunized individuals, HB surface antibody. Unless already known, the infectious status of the source needs to be determined. Unless the source is known to be negative for HBV, HCV, and HIV, post-exposure prophylaxis (PEP) should be initiated, ideally within one hour of the injury.

The Arthus reaction was discovered by Nicolas Maurice Arthus in 1903. Arthus repeatedly injected horse serum subcutaneously into rabbits. After four injections, he found that there was edema and that the serum was absorbed slowly. Further injections eventually led to gangrene.

The distribution of naloxone to injection drug users and other opioid drug users decreases the risk of death from overdose. The Centers for Disease Control and Prevention (CDC) estimates that U.S. programs for drug users and their caregivers prescribing take-home doses of naloxone and training on its utilization are estimated to have prevented 10,000 opioid overdose deaths. Healthcare institution-based naloxone prescription programs have also helped reduce rates of opioid overdose in the U.S. state of North Carolina, and have been replicated in the U.S. military. Nevertheless, scale-up of healthcare-based opioid overdose interventions is limited by providers' insufficient knowledge and negative attitudes towards prescribing take-home naloxone to prevent opioid overdose. Programs training police and fire personnel in opioid overdose response using naloxone have also shown promise in the US.

Permanent brain damage may occur due to cerebral hypoxia or opioid-induced neurotoxicity.

Due to the difficulty of exploring host and amebic factors involved in the pathogenesis of amebic liver abscess in humans, most studies have been conducted with animal models (e.g., mice, gerbils, and hamsters). Histopathological findings revealed that the chronic phase of amebic liver abscess in humans corresponds to lytic or liquefactive necrosis, whereas in rodent models there is granulomatous inflammation. However, the use of animal models has provided important information on molecules and mechanisms of the host/parasite interaction in amebic liver abscess.

Risk factors for opioid overdose include opioid dependence, injecting opioids, using high doses of opioids, and use together with alcohol or benzodiazepines. The risk is particularly high following detoxification. Dependence on prescription opioids can occur from their use to treat chronic pain.

Sneezing generally does not present any particular risks to the individual, and is usually more of an annoyance than a risk of injury. The fits of sneezing brought about by the photic sneeze reflex can, however, have dangerous implications during certain scenarios and activities, such as operating a vehicle, or while undergoing operations (dental, optical) and having bright lights directed towards the patient's face.