HOW LONG DOESy 21 xanax REMAIN IN YOUR SYSTEM? Whether or not you buy y 21 xanax depends on your own choice, but the half-life of the drug depends on several factors like how long the medication was taken, the patient’s metabolic rate, the patient’s age, health, and weight, etc. All these factors must be considered at least once if you buy a prescribed dosage of the drug so that you can know about the half-life of the drug or how long it will remain in your system.