Final answer:
A harmful and undesired effect secondary to the main effect of a medication is known as a side effect, which can range from mild to severe. Superinfections and drug tolerance are examples of adverse effects, and the reaction to a drug can be influenced by the placebo effect.
Step-by-step explanation:
A harmful and undesired effect secondary to the main or therapeutic effect of a medication is called a side effect. Side effects can vary widely in severity and duration, from mild and temporary to severe and long-lasting. For instance, use of broad-spectrum antibiotics can lead to a superinfection, which is a secondary infection that occurs because the antibiotics disrupt the body's normal flora, allowing resistant organisms to overgrow. Moreover, over time, a patient may develop a tolerance to a drug, meaning that the individual requires more and more of the drug to experience the same effects. This is distinct from drug dependence, where stopping the drug leads to physical or psychological symptoms. It is also crucial to understand that not all responses to medications are due to their direct pharmacological effects; the placebo effect is a psychologically based reaction that may occur simply because the individual believes they are being treated.