Funny use of the word 'synthetic' to add extra spookiness. Something like 60% of first-world medicines are natural origin or secondary metabolites of natural origin - the other 40% are all synthetic. [1]
[1] https://www.spandidos-publications.com/10.3892/br.2017.909
https://en.wikipedia.org/wiki/Opioid#Semisynthetic_and_synth...
I am not a doctor: Of the very powerful "powder" drugs heroin is the safest as it has very little effect on the involuntary respiratory system which is the route that opiates take to kill. "Nodding off" on synthetic opiates leads commonly to death (which is why you always prod a sleeping junky - wake up!!) but not so much on heroin.
But it is much safer to smoke opium, if you wish to have such a habit. Much less chance of death and disease.
So, you cruel, nay, sadistic lawmakers: Legalise opium!
But people popping pills are nearly invisible and die quietly - people smoking opium fill up establishments, make funny smells and do not conveniently die off in the corner.....
Natural opioids: codeine, morphine.
Semi-synthetic opioids: heroin, hydromorphone, hydrocodone, oxycodone, etc.
Synthetic opioids: methadone (though this is usually excluded from the reporting of "synthetic opioid overdose deaths" for various reasons), demerol, fentanyl, lots of -fentanyl analogs, etc.