No, I'd consider it one of the easier addictions to have, but it really depends on what metrics you're talking about. Hardest to quit? Maybe, but subjective.
Alcoholism is hands down probably the worst, IMO.
Seconded. If you have money and means, you can live a relatively normal life as an Opioid addict. There are many elements to the psychology of addiction that I only understood when I arrived at a place in which I had full control over my intake. When I was in the Middle East and Asia, I could essentially use as much as I wanted to use. When I first got there, I was salivating and went crazy. It's like being a kid. Your mom might not allow soda at your house, so when you go to your friend's house you drink so much soda that you get sick. You're still drinking that soda even though you really don't even want it. Your past experience of not having it is telling you to take as much as you can while the gettting's good.
This was the same when I got to Cambodia. I would use 5 grams of Heroin a day for the first couple of months. This was pretty potent Heroin too. I remember just sitting for hours in my room, staring at the wall. I wasn't high. I wasn't not high. I was just completely numb to everything. It didn't feel bad and it didn't feel good. It was not what I thought it would ever look like if I had all the dope I could want. The most insane thing happened in that I decided on my own volition to reduce the dosage. At the end of my contract, I was using a still-hefty two grams of Heroin per day intravenously, but this was enough for me to still get enjoyment out of the drug. I'm sure you know what it's like to smoke so much weed that you don't even get high anymore. It was basically like that.
I'm kind of digressing though. Opiates largely leave your higher executive functions intact. You can't say the same for say, Methamphetamine, Crack Cocaine or Alcohol. The part of Heroin that "steals your soul" comes only when you run out of Heroin and your reptilian brain takes over. Using Opioids is a lot like other habits, say, eating three meals a day. I would generally do 3 shots per day. One when I woke up, one right at the end of my lunch break before afternoon classes and then one right around 8:00 before I crashed. When money wasn't an issue, it seemed remarkably easy to have an Opiate habit and live a normal functional life. I was a teacher and both my supervisors and the kids seemed to enjoy me.
If I were in the United States, it might be different. Money is more of an issue. The cops are more of an issue. People are going to be looking you over much more closely as a teacher in the states. The Cambodian police never would have arrested me knowing I'm a teacher in good standing, there to help their people, for just a small amount of Heroin. At worst, you can whip out a 100 dollar bill and not only get off, but make fast friends with the arresting officer in the process.
There is a major history of Opiate addiction among medical doctors. I think for the most part, those days are over, but think about it, you could be a great, well-respected, well-educated medical doctor and it wouldn't at all be crazy to be shooting up Morphine several times a day in between patients. That's a case study I often look to when people ask questions like this.
Opioids in general are non-toxic to the body. That is, they do not cause damage. The most dangerous side effect commonly experienced is severe constipation, but that can be managed in almost all cases. The poor health of Heroin addicts is often more a result of hard living and forgoing all other necessities in order to afford Heroin.
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My belief is that Methamphetamine is by far the worst addiction/dependence out there. It ostracizes the user from society pretty quickly, as they are no longer operating in the same reality as everyone else. Methamphetamine is known to be a potent neurotoxin and the damage it does is often permanent in many ways. If I had to put it simply, the difference is that Methamphetamine addiction is like Opioid addiction, in that it is consuming, but the former also drives you fully insane, sometimes irreparably so.