half life , duration, dosing interval

[zongsheng0621]

I'm really confused about this.

alprazolam half life is roughly 12 hours, and the dosing interval is 3 times daily.
so I think its duration is 8 hours

another drug fludiazepam half life is roughly 23 hours, and the dosing interval is still 3 times daily
its duration is also 8 hours ?

for bzd drugs, it is hard to find complete data about their duration of action, the half-life and the dosing interval

question is how do I consider a drug is long acting or not ?

the previous example, should I say fludiazepam duration equals to alprazolam ? even its half life is twice as alprazolam.

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[KidPharmD]

Half life is only partially related to duration since duration is based on when drug levels fall below an effective level. That effective level might be 50% of peak (duration = half life) or it might be 80% or even 90% or peak levels.

The definition of long acting depends on the drug and class. I would say that drugs that are long acting have significantly longer duration that others of that class (which may or may not mean a longer dosing interval).

 

[Heroic Combo]

I was always mystified with diazepam. The half life is 33 hours. However, people take doses such as 10 mg TID in some cases. Does the blood level just keep compounding on top of itself? I suppose the dose keeps stacking and any BID or TID diazepam regimen has its own built-in uptitration (blood levels keep increasing without dose increasing) without changing the dose? I'm sure there's a ceiling somewhere as far as blood levels go and as metabolic rate nears the top end. I guess nobody has ever given me a good explanation here.