Most rashes are minor and settle on their own, but a few patterns signal an emergency. Get urgent help if a rash spreads very quickly over hours, covers a large part of the body, blisters or peels (especially around the eyes, mouth or genitals), or appears alongside a high fever, severe headache, neck stiffness or confusion.
A key red flag is a rash made of small purple or red spots that do not blanch (fade) when you press a clear glass firmly against them. This non-blanching rash can be a sign of meningococcal infection or a blood-clotting problem and is a medical emergency. Call your local emergency number rather than waiting.
Also treat swelling of the lips, tongue, face or throat, widespread hives with wheezing or difficulty breathing, or a feeling of faintness as a possible severe allergic reaction (anaphylaxis), which needs emergency treatment immediately. For rashes without these features, a doctor or pharmacist can usually advise within a day or two. This is general information, not a diagnosis, so contact a clinician about your specific situation.