Final answer:
Contra-actions are undesirable or adverse reactions that may occur during or after a treatment, such as allergic reactions or the need for increased amounts of a drug to achieve the same effect (tolerance).
Step-by-step explanation:
Reactions that may happen during or after a treatment are known as contra-actions. These reactions can be both positive and negative. However, in the context of the question and the provided information, contra-actions are generally considered to be undesirable or adverse effects.
For example, allergic reactions are a type of contra-action where a person's immune system reacts to a harmless protein from the environment, causing symptoms such as watery, itchy eyes, and sneezing. In the case of drug treatments, genetic and environmental factors may influence whether a person experiences an adverse reaction. Furthermore, treatment for certain reactions, like type III hypersensitivity reactions, typically includes anti-inflammatory steroid treatments or antihistamine treatments.
Tolerance, where a user requires increasing amounts of a drug to experience the same effects, is an example of a negative consequence of drug use which could also be classified as a type of contra-action.