One reason why is that addicts take a lot of barbiturates for a long time. And that causes a lot of problems. So are bad judgment, bad reflexes, and fatigue. On the physical side, there's deterioration of the central nervous system and damage to the liver and pancreas, both linked to the toxicity of the drug in the body. Then there's addiction.
Tolerance builds quickly.
That means users have to keep raising their dosage to keep feeling the way they felt yesterday and the day before that. And that can mean disaster. And if you think it can't happen to you because you're not that stupid, think again. Everybody who gets strung out on barbiturates is that stupid. And if you're not that stupid to begin with, don't worry about it.