How long will it take for X drug to leave my system?


I know all about the half life of chemical compounds...basically it's exactly how it sounds. But it turns out I'd been applying it wrong.

I always figured something would be basically out of the system in 2 half lives, but it turns out, it's actually more like 5!

Wikipedia defines it:

The biological half-life or terminal half-life of a substance is the time it takes for a substance (for example a metabolite, drug, signalling molecule, radioactive nuclide, or other substance) to lose half of its pharmacologic, physiologic, or radiologic activity, according to the Medical Subject Headings (MeSH) definition.[1]

Typically, this refers to the body's cleansing through the function of kidneys and liver in addition to excretion functions to eliminate a substance from the body.

In a medical context, half-life may also describe the time it takes for the blood plasma concentration of a substance to halve (plasma half-life) its steady-state.

The relationship between the biological and plasma half-lives of a substance can be complex depending on the substance in question, due to factors including accumulation in tissues (protein binding), active metabolites, and receptor interactions.[2]

So something like Flexeril, which has a half life of 18-20 hours will actually take something like 4 days to totally clear your system...crazy, right?

Who Me?

Well-Known Member
Isn't it something like half goes out in the half life. Then another half goes out in another half life. Etc.


Active Member
Isn't it something like half goes out in the half life. Then another half goes out in another half life. Etc.

Yes, that's exactly how it works. And though theoretically, you never get to zero this way, in practice, the drug is down to inconsequential amounts after about four half lives.

There is one further complication, though. The half life in the blood may be less than the half life in the tissues, and even different tissues may have different half lives. So you have to know which half life you're interested in.

For example, magnesium is cleared fairly quickly from the blood, generally in a matter of hours. However, the half life of magnesium in tissues is anywhere from 41 to 181 days, depending on the tissue.

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