Action of a drug other than that desired for beneficial pharmacological effect.

a secondary effect of a drug which is produced as well as the therapeutic effect. The term is usually used to refer to undesirable or toxic effects.

a toxic effect from the drug which is not wanted

a toxic effect, which is not wanted, from taking the drug

An additional activity of a drug that may produce detrimental or unwanted effects

A predicate that produces a side-effect is one that has any effect on the “outside world” (the user's terminal, a file, etc.), or that changes the Prolog database.