Which is one reason most opiates, and most other drugs you can easily overdose on and die (as opposed to water), are not legal to buy without prescription.
Actually, no, the reason why you can't buy opiates without a prescription is because they are addictive psychoactives.
You can very easily overdose on Tylenol (acetaminophen, paracetamol), but it will take you a while to die from liver failure. You can buy unlimited amount of Tylenol in every pharmacy.
Over at Scott Adams' Blog you can find a very fine example of using the 'Rationality Engine' to solve the social problem of assisted dying.