Affinity in pharmacology refers to:

Affinity in pharmacology specifically refers to the strength of attraction between a drug and its receptor. This concept is critical in understanding how effectively a drug can bind to a receptor, which directly influences that drug's efficacy in producing a biological response. A higher affinity means that the drug binds more tightly to the receptor, potentially leading to a stronger or more prolonged effect.

In contrast, the amount of drug required for an effect pertains to the drug's potency, not its affinity. Potency describes how much drug is needed to produce a given effect and does not directly measure the strength of the binding interaction. The duration a drug remains in the system is related to pharmacokinetics, particularly the drug's half-life, but does not address affinity. Lastly, the likelihood of a drug causing side effects involves the drug’s profile and interactions within the body, which are distinct from the concept of affinity and receptor binding. Thus, affinity is solely focused on the interaction strength between a drug and its specific receptor.