Final answer:
Any action of a drug other than the desired one is referred to as a side effect or adverse effect. Drug abuse and potential overdoses occur when drugs are used without medical advice or in too high doses, leading to drug addiction. Psychoactive drugs, whether agonists or antagonists, alter brain chemistry and can be addictive.
Step-by-step explanation:
Any action of a drug other than the desired one is typically referred to as a side effect or an adverse effect. Prescription or over-the-counter medicines taken in doses that are too high can lead to overdoses. Similarly, overdoses of illegal drugs can occur with excessive use or when mixing substances. Drug abuse is characterized by the use of a drug without the advice of a medical professional and for reasons not originally intended. This can lead to drug addiction, which is the compulsive use of a substance such as a psychoactive drug despite negative consequences that such use may entail.
Psychoactive drugs can have a mechanism of action that affects brain chemistry, which in turn may cause changes in mood, thinking, perception, and behavior. These drugs act either as agonists, enhancing the activity of particular neurotransmitters, or as antagonists, decreasing the activity of particular neurotransmitters. Misuse of these drugs, whether legal or illegal, can lead to addiction. Sustained use not only poses the risk of addiction but also can require higher doses to achieve the same effect, potentially leading to an overdose.