Risk factors for the development of acne, other than genetics, have not been conclusively identified. Possible secondary contributors include hormones, infections, diet and stress. Studies investigating the impact of smoking on the incidence and severity of acne have been inconclusive. Sunlight and cleanliness are not associated with acne.

The relationship between diet and acne is unclear, as there is no high-quality evidence that establishes any definitive link between them. High-glycemic-load diets have been found to have different degrees of effect on acne severity. Multiple randomized controlled trials and nonrandomized studies have found a lower-glycemic-load diet to be effective in reducing acne. There is weak observational evidence suggesting that dairy milk consumption is positively associated with a higher frequency and severity of acne. Milk contains whey protein and hormones such as bovine IGF-1 and precursors of dihydrotestosterone. These components are hypothesized to promote the effects of insulin and IGF-1 and thereby increase the production of androgen hormones, sebum, and promote the formation of comedones. Effects from other potentially contributing dietary factors, such as consumption of chocolate or salt, are not supported by the evidence. Chocolate does contain varying amounts of sugar, which can lead to a high glycemic load, and it can be made with or without milk. Few studies have examined the relationship between obesity and acne. Vitamin B may trigger skin outbreaks similar to acne (acneiform eruptions), or worsen existing acne, when taken in doses exceeding the recommended daily intake.

Oil production in the sebaceous glands increases during puberty, causing comedones and acne to be common in adolescents. Acne is also found premenstrually and in women with polycystic ovarian syndrome. Smoking may worsen acne.

Oxidation rather than poor hygiene or dirt causes blackheads to be black. Washing or scrubbing the skin too much could make it worse, by irritating the skin. Touching and picking at comedones might cause irritation and spread infection. It is not clear what effect shaving has on the development of comedones or acne.

Some, but not all, skin products might increase comedones by blocking pores, and greasy hair products (like pomades) can worsen acne. Skin products that claim to not clog pores may be labeled noncomedogenic or non-acnegenic. Make-up and skin products that are oil-free and water-based may be less likely to cause acne. It is not known whether dietary factors or sun exposure make comedones better, worse or have no effect.

A hair that does not emerge normally can also block the pore and cause a bulge or lead to infection (causing inflammation and pus).

Genes may play a role in the chances of developing acne. Comedones may be more common in some ethnic groups. People of recent African descent may experience more inflammation in comedones, more comedonal acne, and earlier onset of inflammation.

Although the exact cause of feline acne is unknown, some causes include:

- Hyperactive sebaceous glands

- Poor hygiene

- Stress

- Developing secondary to fungal, viral, and bacterial infections

- Reaction to medication

- Drinking from plastic containers to which the cat is allergic

- Demodicosis or mange, causing itchiness and hair loss

- Suppressed immune system

- Hair follicles that don't function properly

- Rubbing the chin (to display affection or mark territory) on non-sanitized household items

- Hormonal imbalance

- Contracting the infection from other cats in the same household

Pimple-popping, or Zit-popping, is the act of bursting or popping pimples with one's finger. Pimple-popping can lead to the introduction of bacteria into the pimple, infection, the creation of more pimples, and permanent scarring. Thus, popping is usually deprecated by dermatologists and estheticians and it is recommended to let the pimples run through their life span.

Practicing good hygiene, including regularly washing skin areas with neutral cleansers, can reduce the amount of dead skin cells and other external contaminants on the skin that can contribute to the development of pimples. However, it is not always possible to completely prevent pimples, even with good hygiene practices as a number of externalities such as hormones and genetics are at play.

Tropical acne is unusually severe acne occurring in the tropics during seasons when the weather is hot and humid.

Comedones are associated with the pilosebaceous unit, which includes a hair follicle and sebaceous gland. These units are mostly on the face, neck, upper chest, shoulders and back. Excess keratin combined with sebum can plug the opening of the follicle. This small plug is called a microcomedo. Androgens increase sebum (oil) production. If sebum continues to build up behind the plug, it can enlarge and form a visible comedo.

A comedo may be open to the air ("blackhead") or closed by skin ("whitehead"). Being open to the air causes oxidization, which turns it black. "Propionibacterium acnes" is the suspected infectious agent in acne. It can proliferate in sebum and cause inflamed pustules (pimples) characteristic of acne. Nodules are inflamed, painful deep bumps under the skin.

Comedones that are 1 mm or larger are called macrocomedones. They are closed comedones and are more frequent on the face than neck.

Solar comedones (sometimes called senile comedones) are related to many years of exposure to the sun, usually on the cheeks, not to acne-related pathophysiology.

Hyperpigmentation can be caused by sun damage, inflammation, or other skin injuries, including those related to acne vulgaris. People with darker skin tones are more prone to hyperpigmentation, especially with excess sun exposure.

Many forms of hyperpigmentation are caused by an excess production of melanin. Hyperpigmentation can be diffuse or focal, affecting such areas as the face and the back of the hands. Melanin is produced by melanocytes at the lower layer of the epidermis. Melanin is a class of pigment responsible for producing colour in the body in places such as the eyes, skin, and hair. As the body ages, melanocyte distribution becomes less diffuse and its regulation less controlled by the body. UV light stimulates melanocyte activity, and where concentration of the cells is greater, hyperpigmentation occurs. Another form of hyperpigmentation is post inflammatory hyperpigmentation. These are dark and discoloured spots that appear on the skin following acne that has healed.

Hyperpigmentation is associated with a number of diseases or conditions, including the following:

- Addison's disease and other sources of adrenal insufficiency, in which hormones that stimulate melanin synthesis, such as melanocyte-stimulating hormone (MSH), are frequently elevated.

- Cushing's disease or other excessive adrenocorticotropic hormone (ACTH) production, because MSH production is a byproduct of ACTH synthesis from proopiomelanocortin (POMC).

- Acanthosis nigricans—hyperpigmentation of intertriginous areas associated with insulin resistance.

- Melasma, also known as "chloasma"—patchy hyperpigmentation

- Acne scarring from post-inflammatary hyperpigmentation

- Linea nigra—a hyperpigmented line found on the abdomen during pregnancy.

- Peutz-Jeghers syndrome—an autosomal dominant disorder characterized by hyperpigmented macules on the lips and oral mucosa and gastrointestinal polyps.

- Exposure to certain chemicals such as salicylic acid, bleomycin, and cisplatin.

- Smoker's melanosis

- Coeliac disease

- Cronkite-Canada syndrome

- Porphyria

- Tinea fungal infections such as ringworm

- Haemochromatosis—a common but debilitating genetic disorder characterized by the chronic accumulation of iron in the body.

- Mercury poisoning—particularly cases of cutaneous exposure resulting from the topical application of mercurial ointments or skin-whitening creams.

- Aromatase deficiency

- Nelson's syndrome

- Grave's disease

- As a result of tinea cruris.

Hyperpigmentation can sometimes be induced by dermatological laser procedures.

In stage III disease, fistulae left undiscovered, undiagnosed, or untreated, can lead to the development of squamous cell carcinoma, a rare cancer, in the anus or other affected areas. Other stage III chronic sequelae may also include anemia, multilocalized infections, amyloidosis, and arthropathy. Stage III complications have been known to lead to sepsis, but clinical data is still uncertain.

Several triggering factors should be taken into consideration:

- Obesity is an exacerbating rather than a triggering factor, through mechanical irritation, occlusion, and maceration.

- Tight clothing, and clothing made of heavy, non-breathable materials.

- Deodorants, depilation products, shaving of the affected area – their association with hidradenitis suppurativa is still an ongoing debate amongst researchers.

- Drugs, in particular oral contraceptives (i.e., oral hormonal birth control; "the pill") and lithium.

- Hot and especially humid climates (dry/arid climates often cause remission).

Feline acne is a problem seen in cats primarily involving the formation of blackheads accompanied by inflammation on the cat's chin and surrounding areas that can cause lesions, alopecia, and crusty sores. In many cases symptoms are mild and the disease does not require treatment. Mild cases will look like the cat has dirt on its chin, but the dirt will not brush off. More severe cases, however, may respond slowly to treatment and seriously detract from the health and appearance of the cat. Feline acne can affect cats of any age, sex or breed, although Persian cats are also likely to develop acne on the face and in the skin folds. This problem can happen once, be reoccurring, or even persistent throughout the cat's life.

Sebaceous glands are skin glands that produce oil and are mostly found in the skin of the chin, at the base of the tail, and in the eyelids, lips, prepuce, and scrotum. They are connected to hair follicles. In acne, the follicles become clogged with black sebaceous material, forming comedones (also known as blackheads). Comedones can become irritated, swollen, infected, and ultimately pustules. These may elicit itching and discomfort due to swelling and bacterial growth inside infected glands. Cats may continue to scratch and reopen wounds, allowing bacterial infections to grow worse. Bacterial folliculitis occurs when follicules become infected with "Staphylococcus aureus", and commonly associated with moderate-to-severe feline acne. Secondary fungal infections (species "malassezia") may also occur.

Other conditions that can cause similar-appearing conditions include skin mites, ringworm, yeast infection, or autoimmune diseases such as eosinophilic granuloma complex ("rodent ulcers"). These can be ruled out by a simple biopsy of affected cells.

Feline acne is one of the top five most common skin conditions that veterinarians treat.

Women, especially those who are menopausal, are more likely than men to develop rosacea.

Hyperpigmentation is the darkening of an area of skin or nails caused by increased melanin.

Excoriated acne (also known as Picker's acne or Acne excoriée des jeunes filles) is a mild acne accompanied by extensive excoriations.

Oil acne is an occupational skin condition caused by exposure to oils used in industry.

Treatment should be sought immediately in order to avoid hospitalization. If not treated, hospitalization for an extended period of time (usually two weeks) is likely. During hospitalization, the patient is tested for signs of system degradation, especially of the skeletal structure and the digestive tract. By this time open sores will develop on the upper torso. Some will be the size of dimes, others will be large enough to stick a couple fingers into. They will crust up, causing cohesion to any fabric the sores touch, which is extremely painful to remove. It is recommended to sleep on one's sides until the cystic condition subsides, in order to avoid any uncomfortable situations. Debridement and steroid therapy is preferred over antibiotics. Recurrent AF is extremely rare. Bone lesions typically resolve with treatment, but residual radiographic changes, such as sclerosis and hyperostosis, may remain. Scarring and fibrosis may result from this acute inflammatory process.

The disease activates at the height of puberty, usually at around 13 years of age. Acne fulminans predominantly affects young males aged 13 to 22 years with a history of acne.

Small intestinal bacterial overgrowth (SIBO) was demonstrated to have greater prevalence in rosacea patients and treating it with locally acting antibiotics led to rosacea lesion improvement in two studies. Conversely in rosacea patients who were SIBO negative, antibiotic therapy had no effect. The effectiveness of treating SIBO in rosacea patients may suggest that gut bacteria play a role in the pathogenesis of rosacea lesions.

In 1958, at a meeting of the Detroit Dermatological Society, Burns and Colville presented a 16-year-old white boy with acute febrile disease and acne conglobata. Many similar cases have been reported since then. Genetic factors may play an important role in some patients; 4 sets of identical twins who developed an identical pattern of acne fulminans have been documented.

Acne keloidalis nuchae (also known as "Acne keloidalis", "Dermatitis papillaris capillitii", "Folliculitis keloidalis", "Folliculitis keloidis nuchae", and "Nuchal keloid acne") is a destructive scarring folliculitis that occurs almost exclusively on the occipital scalp of people of African descent, primarily men. This is mainly because men often cut their hair very low as opposed to women, allowing the hair to prick the occipital scalp and upset it. Acne keloidalis nuchae most commonly presents itself in individuals aged 13 to 25. The disease is closely related to pseudofolliculitis barbae and both occur frequently in black men in the military, where it is so common that services often have widely known protocols for management.

Steroid acne is an adverse reaction to corticosteroids, and presents as small, firm follicular papules on the forehead, cheeks, and chest. Steroid acne presents with monomorphous pink paupules, as well as comedones, which may be indistinguishable from those of acne vulgaris. Steroid acne is commonly associated with endogenous or exogenous sources of androgen, drug therapy, or diabetes and is less commonly associated with HIV infection or Hodgkin's disease.

Fiddler’s neck does not usually form unless the musician is practicing or playing for more than a few hours each day, and only seems to develop after a few years of serious playing. Thus, when not infected or otherwise problematic, fiddler’s neck may be known as a benign practice mark and may be worn proudly as an indication of long hours of practice. Blum & Ritter (1990) found that 62% of 523 professional violinists and violists in West Germany experienced fiddler’s neck, with the percentage among violists being higher (67%) than among violinists (59%). Viola players are believed to be more predisposed to developing fiddler’s neck than violinists because the viola is larger and heavier, but this has not been empirically confirmed.

The development of fiddler’s neck does not depend on preexisting skin problems, and Blum & Ritter find that only 23% of men and 14% of women in their study reported cutaneous disorders in other parts of the face (mainly acne and eczema) that were independent of playing the violin or viola. Fiddler’s neck may exacerbate existing acne, but acne may also be limited solely to the lesion and not appear elsewhere. Nonetheless, musicians with underlying dermatologic diseases like acne and eczema are more endangered by fiddler’s neck than others. Males may develop folliculitis or boils due to involvement of beard hair.

Tar acne is an occupational skin condition caused by exposure to tars used in industry.

The Mayo Clinic suggests the following: antibiotics (generally the lowest side effect profile compared to other treatments); corticosteroids (e.g., prednisone); but corticosteroids have many side effects, including "moon face" for the duration of the medication's trial usage, as well as unwanted hair growth for females and/or osteoporosis with long-term use. Tumor necrosis factor (TNF)-alpha inhibitors like infliximab (Remicade) and adalimumab (Humira) have shown promise for some, but they should probably be considered a third-line treatment, as treatment is associated with increased risk of infection, heart failure and certain cancers. Surgery is also available for those overwhelmed by the condition, but it will not cure the condition, just relieve the skin-related issues for a while. The disease is pernicious and is almost always guaranteed to return, if not in the same spot where the surgery was performed.

Some products for adult acne may help relieve some symptoms for hidradenitis sufferers, although there is no guarantee it will work in all or even most individuals. Birth control Medication may relieve some symptoms for women; there is a hormonal treatment for men as well but that has not proven to be safe or effective as of yet.

Alternative treatments include alpha hydroxy acids (naturally available in small amounts in citrus fruits), Azelaic acid, and zinc. It is not thought that they are as effective as standard medical treatment but they tend to have less side effects. Some suggest tea tree oil and a specific strain of brewer's yeast, called CBS 5926. However, the former can cause contact dermatitis for some as well as breast development in teenage boys and should not be used if one suffers from rosacea as well due to it making the symptoms of that strand of acne worse; the latter (CBS 5962) can cause migraines and intestinal issues for some. None of these have been formally tested by the FDA so beware the risk you take when you forgo traditional Western medicine for herbal supplements.

Cosmetics play an important role as causal factors for perioral dermatitis. Regular generous applications of moisturising creams cause persistent hydration of the horny layer causing impairment and occlusion of the barrier function, irritation of the hair follicle and proliferation of skin flora. Combining this with night cream and foundation significantly increases risk of perioral dermatitis by 13-fold.