There is no simple answer to how much Xanax (a type of benzodiazepine) is a lethal dose. Each person's body is different, so the way the powerful narcotic is processed on a person to person basis is also different. The chances of overdose are accelerated by other intoxicants consumed, chiefly alcohol. All prescription drugs come with directions written on the bottle and are to be followed strictly at all times. If you think you or someone you know may have taken a higher than prescribed amount of Xanax or other medication, call poison control or 911 immediately.