Final answer:
A side effect is an undesirable or harmful unintended action of a medication, which can happen with misuse or due to individual reactions to the drug.
Step-by-step explanation:
A side effect is an unfavorable or harmful unintended action of a medication. This can occur with both prescription or over-the-counter medicines when taken in doses higher than recommended, or due to an individual’s unique reaction to the medication. Side effects can also arise from overdoses of illegal drugs. While some medications come with minor side effects, others can lead to more severe reactions. It's important to distinguish this from drug abuse, which is the use of a drug without the advice of a medical professional and for reasons not intended by the prescriber. Another serious concern is drug addiction, described as the compulsive use of substances such as psychoactive drugs despite the negative consequences that such use may entail.