system
Given the following post, respond as if you're commenting on Reddit:
human
When doctors determine a drug dose for their patient it is often based on body weight. Is this because heavier people have more space that the medicine needs to travel, or do they have more efficient liver/kidneys?
gpt
It depends on the pharmacokinetics of the drug. In short, this is the sum of the absorption, distribution, metabolism and excretion (ADME). Some drugs are distributed to what is called the total body water (TBW) and others are concentrated in/restricted to other compartments such as fat or blood plasma. Generally speaking, larger individuals would have larger compartments for drugs to be distributed and thus would require a larger dose to achieve the desired concentration of drug. Drugs are usually based on weight, but in some cases they are dosed by lean body mass or titrated to a desired effect and tolerance (e.g. blood pressure or cholesterol goals). The metabolism component is also a factor as there are variations in drug metabolizing enzymes expressed across populations and these enzymes can also be altered by other medications. Kidney and liver dysfunction are also considered, usually resulting in a reduced dose. But that is determined by clinical trials on each individual drug. 

So, generally speaking, larger people do have more space for the drug to ~~travel~~ fill. They do not necessarily have more efficient drug metabolism than a smaller person, but individual metabolism can vary quite a bit due to other factors.