So perhaps the poppy also produces a substance that creates an aversion affect (probably due to extreme nausea) when it reaches a certain concentration in the body. If you approach an overdose, you might start puking your guts out, and wouldn't even be able to look at a 1928 O'Keeffe painting without clamping your jaw shut.

With purified morphine or heroin or methadone, you might just fall asleep and never wake up.

But the raw plant material might also contain something you don't know about that erodes the septum in your heart, and you die from chronic use instead of overdose. That's just one of the risks with whole-plant drugs.

Generally speaking, we have attempted to eliminate hidden side effects by using purified molecules, and attempted to overcome overdose by using prescriptions from trained experts. That isn't necessarily the best or only way to run civilization's pharmacy.