But you can protect yourself. Whenever you have a new batch of some street drug, you test with a small dose. Maybe 10% of what you'd normally use. If it's too little, test with maybe twice as much. Waiting as appropriate, if tolerance is an issue (as with LSD). And then use what you need, to get the desired effect.
In most cases, bad drugs are either cut way too much, or adulterated with some other drug. But it's pretty rare for someone to be selling outright poison. So maybe an amphetamine in the MDMA or LSD. But nothing that's likely to kill at 10% of a typical dose.
The one key exception is fentanyl. So yes, some idiot may indeed sell you heroin (or even cocaine) containing too much fentanyl. However, you can still protect yourself by initially testing a small dose.