system
Given the following post, respond as if you're commenting on Reddit:
human
Why don't drug doses (e.g., ibuprofen, allergy medicine) depend on body weight?

It seems that all over-the-counter and prescription drugs have a single prescribed dosage for an adult, regardless of body weight. Intuitively, however, it would seem that the concentration of the active ingredient in the bloodstream of a 95lb woman vs. a 200lb man would be substantially different.

I get that body fat doesn't factor as much into the efficacy of a drug in the bloodstream, but wouldn't blood volume / tissue mass have a major bearing on this?

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gpt
The concentration that the drug reaches *does* depend on body size and composition. In research, it's normal to see doses quoted as milligrams of substance per kilogram of body mass.

But for over-the-counter drugs, manufacturers and regulators don't want to risk a customer calculating the dosage wrong. So they calculate a single compromise number that's large enough to be reasonably effective for the largest customers, but small enough to still be safe for smaller customers.

For some prescription drugs, the same is true. One size of capsule or tablet works for a large enough range of body sizes that it's usually okay to prescribe that size instead of trying to be more specific. Others are available in a range of dosages, and your prescriber will have done the math and chosen the option that's the best match for your body weight.