Relationship between biological half-life and duration of action of a drug?

Question

Half-life describes the time it takes for the blood plasma concentration of a substance to halve its steady-state. First, does this imply that the blood plasma concentration of a drug decrease exponentially? Secondly, what can be said about the change in activity/effectiveness of the drug by time? (I know SSRIs may work long after the drug is off the body)

Answer 1

In order to achieve full efficacy, first a drug must achieve a "steady state" - that is, the amount of drug taken in will be roughly equal to the amount of drug excreted by the body. This normally takes multiple doses to achieve - antibiotics, SSRIs, etc.

There are some drugs that will have efficacy after only a single dose - think pain medications, anti-inflammatories, etc. - but their effects will be heightened by also achieving a "steady state".

To answer the half-life question - in strictest terms, "half-life" would indeed imply that the drug is excreted at an exponential rate, theoretically suggesting that a portion of the drug, however small, would remain within the body in-perpetuity. However, you must also consider that all drugs are metabolized into prodrugs and inactive compounds by the body, which are similarly metabolized and excreted. As such, and taking into account practical thought process, the entirety of a drug will eventually be completely excreted from the body if the drug were not consumed.