why do drugs stay in one’s system for much longer than their effect? For example, diazepam stays in a person’s system 8-10 days, but it’s effects last 4-5 hours? Why the difference?
In: Chemistry
From my understanding, drugs affect the brain by traveling via the bloodstream. As your blood circulates, it’s being filtered by the kidneys, slowly removing the drugs from your bloodstream. Kidneys filter blood by removing waste and extra water to create urine, however this process takes time and also depends how much and often you smoke to expell it all via urine.
Drugs (all kinds including naturally occuring, man made, supplement, prescription drugs, and recreational drugs) need to be at a certain concentration in your blood stream in order to be effective. That specific concentration is different for every drug. Drugs that are more potent are able to have meaningful activity at low concentrations.
This is very different from the concentration needed to be “detectable”. THC, for example, needs to be present at a relatively high level in the blood stream to have effects on the brain and make someone feel “high”. Drugs tests can pick up small levels of THC in your system, so the drug test can be positive for many weeks after your last toke.
One final thing is a pharmacologic principle called volume of distribution. Essentially, some drugs wind up getting distributed in fatty tissue. Which means those drugs could linger at low levels in some people for a long time, depending on how much fat they have. As a result, certain drugs like marijuana can show up as positive on a drug test months after the last exposure.
Simply, the drug concentration drops over time from first dose. But the therapeutic effect depends on that concentration. But statements like “stay in ones system’ usually refer to how low a concentration can still be measured. So if a drug needs to be at a concentration of say, 0.1 mg/liter of blood. but can be measured down to levels as low as 0.0000000001mg/liter. that counts as still in the system, but way,way,way too low to do any action.
Taking medicine and your body using that medicine isn’t a linear process. It’s much more like radioactive decay with a half life. So for example, say the half life is 6 hours, after 6 hours, half of it is metabolized by the body, then after another 6 hours half of that half, so 3/4 of it is gone. Then after another 6 hours, another half goes away, so how 7/8 is gone. Most medicine will be detectable for 4 to 5 half lives, meaning that only about 3-6% is left.
However that doesn’t mean the medicine is effective at treating symptoms that whole time. Theres typically going to be some minimum threshold needed for it to be effective. But that amount is much much more than the amount needed to be detectable.
I just checked, diazepam has a half life of 48 hours. So using the 4-5 half lives then yeah, that lines up with what you were saying that it should stay detectable for 8-10 days. So in diazepam’s case, you need a lot of it in your system for it to be effective, which is why its effects are short.
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