Note that metabolism of drugs is roughly proportional to bodyweight. A petite 100 lb person may only need 200mg for an effective dose. An obese 400 lb person may require ~4x that dosage for the equivalent effect.

The one-size-fits-all drug dosing we do in this country seems crazy to me.

"Research Chemical" drug users have developed techniques to calibrate doses of substances with unknown potency - start with ~100th of what you expect to be an active dose and then gradually double it until proper effects are achieved.

I'd imagine it would be more expensive for pharmacies to distribute drugs in this way, but allowing patients to titrate their doses could significantly reduce/avoid negative side effects from just throwing the same standardized dose at every person with a prescription regardless of bodyweight.