system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do doctors know the amount and strength of medications when prescribing for acute conditions.

When a doctor diagnoses you with something, say a localized infection, how do they determine the amount of the medication as well as the length you take it?
gpt
The general guidelines and typical dose is first determined by pharmaceutical companies who created the drug; and the company get the number through FDA testing/trials to determine the effective dose, allergy, side effects, min/max safe dose, etc.

The prescriber will usually start with the typical dose and adjust if needed… rarely going beyond the recommended max dose so not to damage the liver/kidney/OD/other risks. 

Sometimes a drug will be given for “off label” use and that’s usually determined/backed by research articles. Off label just means the type of use have not been approved by the FDA. 

Source: Im licensed to Rx meds.