Drugs should be highly specific and selective towards their targets in order to maximize their efficacy and minimize the risk of unintended side effects. Drug design is continuously working towards designing more specific and selective drugs that can have more clear-cut effects on more clear-cut targets.
Drug selectivity is defined as the restriction of the activity of the drug molecule towards only one type of target. This target can be anything from a pathogenic enzyme to a receptor in the host organism.
Selectivity is important because it ensures the drug is only acting on the defective or pathogenic molecule. For instance, it is critical that a drug designed to inhibit enzymes be selective for the pathogenic enzyme, and not any host enzyme.
Specificity refers to the capacity of the drug to initiate specific changes at the target. For instance, a drug selective for a receptor can act as the receptor agonist and stimulate its activity, or it can act as a receptor antagonist and inhibit its activity. However, the drug cannot do both, as its mechanism of action must be clearly defined in advance to ensure its safety.