Drug DUI: How long is a drug in your system?
For testing purposes, clients often ask how long a specific drug might be in their system, and if it may show up once the test of the client’s blood or urine takes place.
Drug DUI: How long is a drug in your system – The Half Life
The half-life of a drug is the amount of time it takes the body to eliminate half of the total amount of the drug present in the bloodstream from the body. The term half-life is not synonymous with “duration of effect” which means the length of time that the drug is having its therapeutic effect in the body.
The half life of a medication is the time required for concentrations of the medication to decrease by half; typically serum concentrations. A drug like diazepam has a relatively long half life. Even using the most simplistic pharmacokinetic explanation ( one compartment zero order elimination ) serum concentration would decrease for example from 20 ng / ml to 10 ng/ml in approximately 40 hours ( using a standard half life value).
I have seen references in the literature to diazepam as having a half-life that ranges from 40 to 100 hours, and references to its “Duration of effect” or “Duration of action” as being approximately 3 hours.
Intoxication / DUI and How Long Drugs Are In Your System
That leads to a few questions about how that might relate into intoxication, for DUI or drunk driving purposes:
1. What is the correlation between half-life and duration of effect for a drug like valium? Why is duration of effect so much shorter than the half-life? For example, if it takes the body 40 hours to eliminate half the amount of diazepam present in the body. why doesn’t valium stay active for at least 40 hours?
2. Are there any authoritative sources for diazepam’s “Duration of effect”?
The duration of effect is the time interval while drug concentrations are at or above the concentration at the site of action needed to produce the effect. So in the example of diazepam, depending on the dose, concentrations in the brain needed to produce a specific magnitude of effect will be dependent on the concentrations of diazepam achieved in the brain and the rate of elimination from the brain. For diazepam, because it is very lipophillic ( fat loving) it redistributes from brain to adipose tissue more quickly than the half life of elimination from plasma, hence the shorter duration of effect.
There are also instances where the duration of effect can be longer than the elimination half life from plasma, for example if a parent drug has a short half life but is metabolized to an active metabolite. The duration of effect can be prolonged by the contribution to the effect by the active metabolite.
So there is not usually a simple relationship between simple pharmacokinetic elimination rate and duration of effect.
These are two great questions at the heart of pharmacology. The answers are not simple. As we say to juries, “We are not machine tools.” In essence, the answers depend on the drug and the subject. This results in a large number of variations on a theme, even given the same standard dose to a number of very different people.