Was opium to him tylenol or fentanyl?

Yes. Those are both painkillers, and each has its use in medicine. If your gran was laid up in the hospital with a borken hip, they'd likely give her fentanyl for the pain. Funny enough, fentanyl is just a medical grade heroin (or, as it's also called, opium).

When Marx was writing, cocaine, alcohol, and opium were just about it for modern medicine. Of those, opium had the fewest side effects. It killed your pain and, as long as the dose wasn't extraordinarily high, it didn't kill you. That was it. Compared to alcohol or cocaine, it was by far the ideal treatment. Remember, penicillin wouldn't be discovered until the 20th century -- at this point in history, you'd be lucky if your barber-surgeon was cleaning his blades between patients, let alone washing his hands!