1. A secondary and usually adverse effect of a drug or therapy
2. Any unwanted secondary effect.
An effect, usually undesirable, that accompanies the therapeutic effect for which a particular drug has been administered. Side effects range from mild to severe, and include drowsiness, induced by many antibiotics and antihistamine tablets, and, in the case of some tranquilizers, partial paralysis and blurred vision. Alternative practitioners generally consider side effects to be more significant than do conventional doctors.