1: It slows down your brain for 7 years. The smart become average. Average become stupid. 2: It gives you a whole lot more poisons than tobacco. If you hold it in your lungs it will give you cancer a whole lot sooner.

3: It is unregulated. You might have drugs of unknown origin tainting them. Drugs, pesticides, chemicals that can harm you.

4: A drugged brain is more likely to produce less at work, be more dangerous on the road.

If these aren't good enough answers for you then your drug-saturated brain may be too incapacitated to understand.