Severe allergic reactions can affect any organ or system and they can be caused by any type of allergen. The severe cutaneous reactions due to drugs are drug rash with eosinophilia and systemic symptoms, generalized exfoliative dermatitis, Stevens-Johnson syndrome and toxic epidermal necrolysis. However, the most severe allergic reaction is known as anaphylaxis. It is diagnosed according to clinical indicators which are based on signs and symptoms that let us know the probability of suffering from the disease. Adrenaline is the first-line treatment.