system
Given the following post, respond as if you're commenting on Reddit:
human
How can the dosage of medicine be the same for a 5'3" 105 lb woman, and a 6'4" 240 lb man?

How does the larger man not need double the dose?
gpt
Simply, because it is the most practical approach. For the large majority of drugs on the market, there is insufficient data to establish the correct dose based on the body weight of a person. That said, depending on the drug, doctors will vary dosage of drugs based on several factors, which can include body weight.

Drug development is not quite an exact science, yet: Typically, in early drug discovery, animal experiments are actually performed using a set dose per weight (commonly expressed in mg of drug per kg of body weight). This allows for the determination of pharmacokinetics, pharmacodynamics, efficacy, toxicity, safety margins, etc.... in those animals. Going into the clinic, those data are then plugged into models that provide an estimation of the dosage for humans. Oftentimes, early phases of clinical trials will be used to establish the "best" dose to use (most efficacy, least side effects) based on data from very few to maybe several dozen individuals. Once a dose is determined (typically in mg), that dose will be used to do safety studies in larger groups that are more heterogeneous and closer in weight and height to the general population likely to use the drug. Even those larger studies are often not representative of the entire population the drug will be marketed to as oftentimes, these larger scale trials are performed in countries with lower costs (India or South Africa, for example).

On a more practical level, a safe drug with no adverse side effects is often taken at doses that maximize efficacy in most individuals. In your example, the 105 lb woman could probably take half the dose and still have the same effect because the "regular dose" is more than high enough to saturate the target completely.