I heard synthetic opioids are often very toxic. Fentanyl, ingredient in bad heroin is 10 - 10000 times more toxic than heroin or opium, depends on what strain of fentany it is. Also methadon and polamidon should be much more toxic than heroin or opium. don´t know about vicodin and stuff.
I tried it once 20 years ago, was nice, but isn´t available, where i live.
When you should use it, just use it for some days and come down, and may be start again some days later, oterwise you will get addicted and get the detox...