Vid, page-14

Pharmacokinetics is quite complex . But half life is only one thing to consider in the whole scheme as other factors need to be considered. I will use aspirin as an example

It has a half life of around 15 minutes (dose dependent) but people take it once daily - how so? Because its mechanism of action is long lasting even if the drug is gone. Drugs have different affinities for there receptors (aka its target) - some jump on and off fast while others stick around for a long time so naturally this extends its "action" time.

Hence the potency of a drug depends on its binding affinity for the binding site as well as its binding ability to bring about the desired effects

Another example is something like prednisolone or steroids for that fact - prednisolone has a half life of around 1 hour yet its effect can last days - why - because it triggers an effect that results in altered protein synthesis and this effect last a lot longer than the drug half life.

Pharmacology 101 complete!