Opium is physically addictive. Cocaine is psychologically addictive. And from what I once read in the Merck Manual, Marijuana is apparently neither. What is the difference between physical and psychological addictions to drugs? They certainly both sound just as serious.
Thank you in advance to all who reply 
Speaking as someone who has done a lot of work in psychopharmacology and also had an addiction myself, I can say that this is a very, very complicated question. Actually, no one has the whole answer although there are whole books written on it.
Cocaine is somewhat physically addicted too although you can’t die detoxing from it like you can with say alcohol.
I can give you a specific example of why this question is hard to answer. Alcoholics that have abused alcohol frequently and for a long-time often experience a syndrome called post-acute withdrawal. PAW typically peaks weeks or months after cesation of alcohol and can last over a year. The symptoms include memory impairment, lack of coordination, stress (a jumpy, twitchy state), and emotional instability.
All of these symptoms are caused by problems in the nervous system but do we call them “physical” or “psychological”? Lack of coordination sounds kind of physical. Memory impairment could go either way because we don’t say that Alzheimer’s patients have a “psychological” problem. What do we say about stress when muscle twitching and binding is causing physical pain?
Do you start to see the problem? All of these things were caused by nervous system damage or change and it is likely that their was a physical change that helped promote the addiction while it was active.
The brain is an organ like any other.
If you are a physicalist, there’s no essential difference. One supposes that a practical demarcation is made on the basis of the nature of the withdrawal. Physiological symptoms like fever are treated as indicating physical addiction whereas psychological addiction would relate to “cravings” and mental disquiet. But both types of effects (physiological and psychological) leave marks of the other type.
This chart from here is informative about the addictive potential of various drugs.
I would say that physical addictions are caused by the body developing a need for a chemical found in the drug. For instance, the body naturally produces nicotine, but it starts to produce less when someone starts using tobacco, and a sudden halt will leave their body short of it. Of course, psychological addictions would be one in that you desire the drug enough to use it despite potential consequences. I’m not a physician or anything, but it seems pretty straightforward to me.
Cite? I think you’re confusing nicotine/acetylcholine with opiates/endorphins+morphine. I haven’t heard of nicotine being endogenous.
Marijuana can be very psychologically addicting, given the right person who craves that “feeling”.
Sounds reasonable.
That is not correct technicallyl speaking. The brain and body have a subtype of acetylcholine receptors called nicotinic receptors that happen to bind nicotine if it is used. Nicotine addiction downregulates those receptors so the neurons don’t fire like normal if nicotine is removed.
That still doesn’t seperate out psychological from physical addictions. Some alcoholics can quite literally die if they stop drinking for any length of time. Alcohol is like insulin is to a diabetic for them and I doubt you could call that a psychological addiction. Other drugs create physical addictions to a greater or lesser degree and it is hard to seperate the psychological from the physical addiction.
