Whenever a new drug is introduced there are always rules about how it is to be prescribed. Well, usually.
Some drugs must be taken with food, others need to be taken on an empty stomach. Some drugs must be taken in the morning, others at night. Some drugs are suitable only for adults, others are best avoided by the elderly or by pregnant women.
The rules about when drugs should be taken are put together after the various trials have been performed. Sometimes, the rules change as doctors watch how a drug affects different groups of people.