Help me Understand Half Life

[DixieWrecked]

So it take roughly 5 half lives for a drug to be out of your system. So if I injected 500mg of test cyp it would take roughly a month to be out of my system. But if I inject 100mg of test cyp, it would also take about 1 month to be out of my system. How does this makes sense? Explain to me like I am five please.

 

[Vince]

Testosterone Cyp Half Life

How long does Test Cyp last in the body? Is this the same as "half life"?

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[Vince]

From a clinical standpoint, it is common to assume that a drug is effectively eliminated after 4-5 half-lives. So, in strict pharmacokinetic terms, if we use a half-life of efavirenz of 50 hours, we would conclude that it is eliminated in 250 hours (5 half-lives), or about 10 days.Sep 20, 2000

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[swoops36]

So it take roughly 5 half lives for a drug to be out of your system. So if I injected 500mg of test cyp it would take roughly a month to be out of my system. But if I inject 100mg of test cyp, it would also take about 1 month to be out of my system. How does this makes sense? Explain to me like I am five please.

It doesn’t really mean it’s ‘out of your system’, it just means it declines by 50% at each interval. There’s a point where there’s still some chemical in your system but it’s too small to have an effect. That point would be reached quicker with the 100mg dose than the 500mg dose.

 

[Cataceous]

No need to get into a lot of detail to explain the example. The half-life model says that if you have a certain amount of a drug present at the start, say X, then at some future time you will have some fraction of X left, with the fraction determined by the elapsed time and the half-life parameter, and independent of the starting amount. So if you're comparing starting with 100 mg versus starting with 500 mg, then the remaining amount is always five times greater with the larger starting amount. Suppose the fraction remaining is one percent after one month. Then starting with 100 mg you're left with 1 mg after a month, whereas starting with 500 mg you're left with 5 mg. In either case you're left with the same fraction of the starting amount. In the case of an elapsed time of five half-lives, this fraction is assumed to be small enough to neglect, about three percent.