Every year the World Anti-Doping Agency updates the the Prohibited List. The list contains not just prohibited substances but prohibited methods, such as blood-doping.
Some substances are prohibited at any time and some substances are only prohibited 'in-competition', i.e. on a match day or within 24 hours of a match.
Under the principle of Strict Liability, players are solely responsible for any prohibited substance found in their system whether there was an intention to cheat or not.
Before taking any medication players are advised to check the ingredients on the Global DRO (Drug Reference Online) website, which will confirm whether the substance is permitted or not. Please note that this website will not confirm the safety of dietary supplements. For information on supplements please see the specific section on this website here.