Itchy rash on the neck that appears suddenly

Think back to the hours before the rash appeared. Hair products, perfume, jewellery, a shirt collar, detergent, sunscreen or a workplace substance can cause contact dermatitis where they touch. Heat and sweating can produce prickly heat rash. A new medicine can cause a drug rash, sometimes beyond the neck. Raised fleeting patches may behave like hives, while a scaly patch can fit atopic dermatitis, psoriasis or ringworm. Small clustered marks after outdoor exposure may suggest bites. The exact appearance and whether the rash moves or persists are more useful than the phrase sudden rash alone.

Rinse the neck with lukewarm water and stop any newly used fragrance, hair, skin or laundry product until the cause is clearer. Avoid necklaces, collars and hot showers if they increase irritation. Pat dry, use a bland moisturiser if the skin is intact, and keep nails short to limit damage from scratching. Record the time of onset, take a photograph, and list medicines, supplements, foods and exposures from the previous day. Do not restart a suspected medicine without speaking to its prescriber.

Get urgent help if neck swelling comes with trouble breathing, wheezing, dizziness, faintness, or swelling of the lips, tongue or face. Prompt medical review is also needed for fever, severe pain, pus, purple spots, blistering, peeling skin, or a rash involving the eyes or mouth. If the rash continues to spread, returns repeatedly or remains unexplained, arrange an examination so a clinician can distinguish contact dermatitis from infection or a medicine reaction.

Ask which recent exposure is most plausible and whether the rash resembles contact dermatitis, heat rash, drug rash or ringworm. Bring the product labels or a medicine list. Ask what can relieve itching, which ingredients to avoid, and what changes would mean you should return sooner.