Allergic rash on the body

The rash pattern, speed of onset, medicines, illnesses and skin exposures guide the diagnosis. Breathing problems, faintness or mouth and throat swelling require emergency action.

Hives form raised itchy welts that can fade in one place and appear elsewhere. Contact dermatitis tends to follow exposure to clothing, fragrance, detergent, plants or metal and may become dry, blistered or scaly. Atopic dermatitis often presents with recurring itch and dry inflamed skin. Drug eruptions can spread widely after a medicine is started.

Heat rash favours hot, sweaty areas; intertrigo develops in moist skin folds. Ringworm may make a scaly expanding ring, scabies commonly causes intense itch with characteristic sites, and psoriasis can form sharply defined scaly plaques. These conditions are not interchangeable, and a body rash alone cannot establish allergy.

Photograph the rash in good light and note when it began, where it first appeared, whether individual patches move, and what makes it better or worse. List new medicines, supplements, foods, detergents, cosmetics, clothing and occupational exposures. Do not deliberately re-expose yourself to a suspected trigger.

Use cool compresses, loose clothing and a fragrance-free moisturiser if the skin is dry. Avoid hot showers, scratching and mixing several medicated creams, since this can alter the appearance or aggravate irritation. A pharmacist or clinician can match itch relief or anti-inflammatory treatment to the likely cause and body site.

Call emergency services if a rash occurs with difficulty breathing, wheezing, throat tightness, faintness, confusion, or rapid swelling of the lips or tongue. A rapidly spreading painful rash, blistering or peeling skin, sores in the mouth or eyes, or a purple rash that does not fade when pressed also needs urgent assessment.

Arrange a routine visit when the rash is persistent, recurrent, widespread, disturbing sleep or accompanied by signs of infection such as pus, crusting, fever and increasing warmth. Seek timely advice about any new widespread rash after starting medicine; do not restart a suspected non-essential product to test the reaction.

Ask which features support hives, contact dermatitis, eczema, infection or a medicine reaction; whether testing would change care; and which treatment is safe for the affected areas. Bring the medicine list, product labels and photographs showing how the eruption changed.