The condition varies from a mild, self-limited rash (E. multiforme minor) to a severe, life-threatening form known as erythema multiformer major (or erythema multiforme majus) that also involves mucous membranes.

Consensus classification:

- Erythema multiforme minor—typical targets or raised, edematous papules distributed

- Erythema multiforme major—typical targets or raised, edematous papules distributed acrally with involvement of one or more mucous membranes; epidermal detachment involves less than 10% of total body surface area (TBSiA)

- SJS/TEN—widespread blisters predominant on the trunk and face, presenting with erythematous or pruritic macules and one or more mucous membrane erosions; epidermal detachment is less than 10% TBSA for Stevens-Johnson syndrome and 30% or more for toxic epidermal necrolysis.

The mild form usually presents with mildly itchy (but itching can be very severe), pink-red blotches, symmetrically arranged and starting on the extremities. It often takes on the classical "target lesion" appearance, with a pink-red ring around a pale center. Resolution within 7–10 days is the norm.

Individuals with persistent (chronic) erythema multiforme will often have a lesion form at an injury site, e.g. a minor scratch or abrasion, within a week. Irritation or even pressure from clothing will cause the erythema sore to continue to expand along its margins for weeks or months, long after the original sore at the center heals.

Erythema multiforme major (also known as "erythema multiforme majus") is a form of rash with skin loss or epidermal detachment.

The term "erythema multiforme majus" is sometimes used to imply a bullous (blistering) presentation.

According to some sources, there are two conditions included on a spectrum of this same disease process:

- Stevens–Johnson syndrome (SJS)

- Toxic epidermal necrolysis (TEN) which described by Alan Lyell and previously called Lyell syndrome[5].

In this view, EM major, SJS and TEN are considered a single condition, distinguished by degree of epidermal detachment.

However, a consensus classification separates erythema multiforme minor, erythema multiforme major, and SJS/TEN as three separate entities.

Erythema multiforme (EM) is a skin condition of unknown cause; it is a type of erythema possibly mediated by deposition of immune complexes (mostly IgM-bound complexes) in the superficial microvasculature of the skin and oral mucous membrane that usually follows an infection or drug exposure. It is an uncommon disorder, with peak incidence in the second and third decades of life. The disorder has various forms or presentations, which its name reflects ("multiforme", "", from "" + ""). Target lesions are a typical manifestation. Two types, one mild to moderate and one severe, are recognized (erythema multiforme minor and erythema multiforme major).

Erythema (redness) multiforme (EM) is usually a reaction of the skin and mucous membranes that occurs suddenly. It appears as a symmetrical rash and may include the mucous membrane lesions. This means that the body is sensitive to something that causes the skin and mucous membranes to react. The more common mild form is refer to as EM minor. It consists of a skin rash that involve no more than one mucosal surface. The sudden onset will progress rapidly as symmetrical lesions with circular color changes in some or all of the lesions. Rash will spread towards center or trunk of the body. Evenly distributed bumps on the skin become classic iris or target lesions. They have bright red borders and small white bumps in the center.

The cause of EM appears to be a highly sensitive reaction that can be triggered by a variety of causes. The causes can include bacterial, viral or chemical products, such as antibiotics – specifically penicillins or cephalosporins. This reaction is an allergic reaction and is in no way contagious.

Erythema multiforme minus is sometimes divided into papular and vesiulobullous forms.

Erythema nodosum is characterised by nodules (rounded lumps) below the skin surface, usually on the shins. These subcutaneous nodules can appear anywhere on the body, but the most common sites are the shins, arms, thighs, and torso. Each nodule typically disappears after around two weeks, though new one may continue to form for up to six or eight weeks. A new nodule usually appears red and is hot and firm to touch. The redness starts to fade and it gradually becomes softer and smaller until it disappears. Each nodule usually heals completely without scarring over the course of about two weeks. Joint pain and inflammation sometimes continues for several weeks or months after the nodules appear.

Less common variants of erythema nodosum include:

- Ulcerating forms, seen in Crohn's disease

- Erythema contusiforme, when a subcutaneous hemorrhage (bleeding under the skin) occurs with a erythema nodosum lesion, causing the lesion to look like a contusion (bruise)

- Erythema nodosum migrans (also known as subacute nodular migratory panniculitis), a rare form of chronic erythema nodosum characterized by asymmetrical nodules that are mildly tender and migrate over time.

The rash is composed of small papular lesions, each on a separate reddened base.

The rings are barely raised and are non-itchy. The face is generally spared.

The first signs of erythema nodosum are often flu-like symptoms such as a fever, cough, malaise, and aching joints. Some people also experience stiffness or swelling in the joints and weight loss.

It occurs in less than 5% of patients with rheumatic fever, but is considered a major Jones criterion when it does occur. The four other major criteria include carditis, polyarthritis, Sydenham's Chorea, and subcutaneous nodules. In this case, it is often associated with Group A streptococcal infection, otherwise known as "Streptococcus pyogenes" infection, which can be detected with an ASO titer.

It is an early feature of rheumatic fever and not pathognomonic of it. It may be associated with mild myocarditis (inflammation of heart muscle). It is also seen in conditions like allergic drug reactions, sepsis and glomerulonephritis.

It often occurs as a harbinger of attacks in hereditary angioedema. In this case it may occur several hours or up to a day before an attack.

Ulcerative sarcoidosis is a cutaneous condition affecting roughly 5% of people with sarcoidosis.

Annular sarcoidosis is a cutaneous condition characterized by papular skin lesions arranged in annular

patterns, usually with a red-brown hue.

Fever and a non specific skin eruption – with reddening (erythema) and swelling (oedema) of the skin – are the most common symptoms of NEH. Patients usually present with the skin eruption 1-2 weeks after use of the cytotoxic drug. Sometimes, the skin eruption can be painful. Skin eruptions can be located on the extremities, trunk, and face. Severe lesions are rare, and can mimic cellulitis. Generalised lesions resembling erythema multiforme have been reported.

Morpheaform sarcoidosis is a very rare cutaneous condition characterized by specific cutaneous skin lesions of sarcoidosis accompanied by substantial fibrosis, simulating morphea.

Erythrodermic sarcoidosis is a cutaneous condition and very rare form of sarcoidosis.

Hypopigmented sarcoidosis is a cutaneous condition characterized by areas of hypopigmented skin. It is usually diagnosed in darkly pigmented races and may be the earliest sign of sarcoidosis.

Papular sarcoid is a cutaneous condition characterized by papules, which are the most common morphology of cutaneous sarcoidosis.

Ichthyosiform sarcoidosis is a cutaneous condition resembling ichthyosis vulgaris or acquired ichthyosis, with fine scaling usually on the distal extremities, by caused by sarcoidosis.

Erythema toxicum neonatorum (also known as erythema toxicum, urticaria neonatorum and toxic erythema of the newborn) is a common rash in neonates. It appears in up to half of newborns carried to term, usually between day 2–5 after birth; it does not occur outside the neonatal period.

Erythema toxicum is characterized by blotchy red spots on the skin with overlying white or yellow papules or pustules. These lesions may be few or numerous. The eruption typically resolves within first two weeks of life, and frequently individual lesions will appear and disappear within minutes or hours. It is a benign condition thought to cause no discomfort to the baby.

TEN ultimately results in extensive skin involvement with redness, necrosis, and detachment of the top (epidermal) layer of the skin and mucosa. Before these severe findings develop, people often have a flu-like prodrome, with a cough, runny nose, fever, decreased appetite and malaise. A history of drug exposure exists on average 14 days (ranging from 1–4 weeks) prior to the onset of symptoms, but may result as early as 48 hours if it is a reexposure.

Initial skin findings include red-purple, dusky, flat spots known as macules that start on the trunk and spread out from there. These skin lesions then transform into large blisters. The affected skin can then become necrotic or sag from the body and peel off in great swaths.

While the presence of lesions is the denominator among patients with PNP, the characteristics of the lesions differ. The five clinical presentations of lesions associated with PNP include:

- "Pemphigus-like": Flaccid blister (discrete), crusts over the raw exuding skin lesions

- "Pemphigoid-like": Tense blister(s) on brick red erythema

- "Erythema multiforme-like": Severe polymorphic skin and/or mucous membrane lesions

- "Graft-vs.-host disease-like": Widespread lichenoid eruption with severe mucous membrane involvement

- "Lichen planus-like": Small red flat-topped scaly papules

It is most common that mucous membrane lesions of the oral cavity are presented first. They can involve the oropharynx, nasopharynx, tongue, and vermilion (red portion) of the lips. They are also known to develop in the conjunctiva of the eye, anogenital (perineum) region, and esophagus. Cutaneous lesions tend to follow the onset of mucosal lesions. The blisters often erupt in waves, usually affecting the upper trunk, head, neck, and proximal extremities. Pemphigoid-like lesions are seen more often on the extremities. Lichenoid lesions are more common among children, presenting on the trunk and limbs, ranging from small red scaly papules to extensive violet to brown papules extending to the face and neck. Within the spectrum of lichenoid presentations are wounds that have features of erythema multiforme and graft-vs.-host disease. Scaly lesions on the palms of the hand and soles of the feet have been noted to coincide with the lichenoid lesions. Lesions of varying morphology may present simultaneously and transform from one type to another as the disease progresses.

Urticarial erythema multiforme is an unusual reaction virtually always associated with antibiotic ingestions, characterized by skin lesions that consist of urticarial papules and plaques, some of which clear centrally forming annular lesions, but with no true urticarial lesions.

Id reactions (also known as "disseminated eczema," and "generalized eczema") are types of acute dermatitis developing after days or weeks at skin locations distant from the initial inflammatory or infectious site. They can be localised or generalised. This is also known as an 'autoeczematous response' and there must be an identifiable initial inflammatory or infectious skin problem which leads to the generalised eczema. Often, intensely itchy, the red papules and pustules can also be associated with blisters and scales and are always remote from the primary lesion. It is most commonly a blistering rash with itchy vesicles on the sides of fingers and feet as a reaction to fungal infection on the feet, athlete's foot. Stasis dermatitis, Allergic contact dermatitis, Acute irritant contact eczema and Infective dermatitis have been documented as possible triggers, but the exact cause and mechanism is not fully understood. Several other types of id reactions exist including erythema nodosum, erythema multiforme, Sweet's syndrome and urticaria.

Erythema nodosum is a form of panniculitis characterised by tender red nodules, 1–10 cm, associated with systemic symptoms including fever, malaise, and joint pain. Nodules may become bluish-purple, yellowing, and green, and subside over a period of 2–6 weeks without ulcerating or scarring. Erythema nodosum is associated with infections, including Hepatitis C, EBV and tuberculosis, Crohn's disease and sarcoidosis, pregnancy, medications including sulfonamides, and some cancers, including Non-Hodgkin lymphoma and pancreatic cancer.

Erythema disappears on finger pressure (blanching), while purpura or bleeding in the skin and pigmentation do not. There is no temperature elevation, unless it is associated with the dilation of arteries in the deeper layer of the skin.

Neutrophilic eccrine hidradenitis (NEH) usually is a cutaneous complication of chemotherapy, but it can also occur for other reasons. It consists of fever and non specific skin lesions. It is rare, and self-limited.

Bullous drug reaction (also known as a "bullous drug eruption", "generalized bullous fixed drug eruption", and "multilocular bullous fixed drug eruption") most commonly refers to a drug reaction in the erythema multiforme group. These are uncommon reactions to medications, with an incidence of 0.4 to 1.2 per million person-years for toxic epidermal necrolysis and 1.2 to 6.0 per million person-years for Stevens–Johnson syndrome. The primary skin lesions are large erythemas (faintly discernible even after confluence), most often irregularly distributed and of a characteristic purplish-livid color, at times with flaccid blisters.

Erythema induratum, or "Bazin disease", is a panniculitis on the back of the calves. It was formerly thought to be a reaction to the tuberculum bacillus. It is now considered a panniculitis that is not associated with a single defined pathogen.

Nodular vasculitis is a skin condition characterized by small, tender, reddened nodules on the legs, mostly on the calves and shins. Microscopically there are epithelioid granulomas and vasculitis in the subcutaneous tissue, making it a form of panicullitis. Most of these cases are now thought to be manifestation of tuberculosis and indeed they respond well to anti-tuberculous treatment.

Causes include infection with dermatophytosis, Mycobacterium, viruses, bacteria and parasites. Eczematous conditions including contact allergic dermatitis and stasis dermatitis as well as stitches and trauma have also been associated with id reactions. Radiation treatment of tinea capitis has been reported as triggering an id reaction.

Acute, tender, erythematous plaques, nodes, pseudovesicles and, occasionally, blisters with an annular or arciform pattern occur on the head, neck, legs, and arms, particularly the back of the hands and fingers. The trunk is rarely involved. Fever (50%); arthralgia or arthritis (62%); eye involvement, most frequently conjunctivitis or iridocyclitis (38%); and oral aphthae (13%) are associated features.