Red rash due to an allergy to penicillin. This is a mild example of hypersensitivity, the most important side effect of penicillin. A more extreme form is that of anaphylactic shock, which may be fatal. Persons allergic to one penicillin will be allergic to all types as the basic structure is the same. A rare, serious toxic effect is encephalopathy due to cerebral irritation. This may result from very high doses, and also at normal doses in patients with kidney failure.