system
Given the following post, respond as if you're commenting on Reddit:
human
What determines how a drug is administered (e.g. orally, anally, injected)?
gpt
It is a combination of several factors:

Is the drug broken down by gastric acid and/or digestive enzymes? In some cases that rules out tablet administration, in other cases you can get past it by coating them for release later in the intestines.


First pass metabolism? Drugs absorbed from the intestines (mostly) go through the liver before entering the systemic circulation. For some drugs the liver clears the drug so efficiently from the circulation that you need to increase the dose massively or give it through another route. If it is absorbed through mucosa you can give it nasally, by buccal/subling adm or per rectum. If it is not absorbed mucosally you will need to give it intravenously or intramuscularly.


Other considerations also come into play. For example for anaesthesia we want to control the pharmacokinetic aspects closely (that is the timing and precise control of drug concentration in the blood stream and CNS) to be able to do this we have to use intravenous access or inhalation (for anaesthetic gasses). 


If you are interested in details you can try to get hold of an introduction to pharmacology, which is the field that deals with this in depth.