This class of drug is a prophylactic for malaria. It, or one of its equivalents, is commonly given to you if you tell your doctor that you are traveling to a part of the world where malaria is currently endemic. It isn't the kind of drug where they worry much about abuse or misuse, as it is cheap and ubiquitous in the developing world.

A doctor can prescribe it. Best to take it under the supervision of a doctor.

It's commonly used for malaria. A related '-quine' is the active ingredient of Gin and Tonic.

GPs won't prescribe on the basis that you might get sick and require a particular medication. You can't walk into your GP's office and ask for antibiotics, for instance.