Whether you sniff it smoke it eat it or shove it up your ass the result is the same: addiction.

The question is frequently asked: Why does a man become a drug addict?
The answer is that he usually does not intend to become an addict. You don’t wake up one morning and decide to be a drug addict. It takes at least three months’ shooting twice a day to get any habit at all. And you don’t really know what junk sickness is until you have had several habits. It took me almost six months to get my first habit, and then the withdrawal symptoms were mild. I think it no exaggeration to say it takes about a year and several hundred injections to make an addict.
The questions, of course, could be asked: Why did you ever try narcotics? Why did you continue using it long enough to become an addict? You become a narcotics addict because you do not have strong motivations in the other direction. Junk wins by default. I tried it as a matter of curiosity. I drifted along taking shots when I could score. I ended up hooked. Most addicts I have talked to report a similar experience. They did not start using drugs for any reason they can remember. They just drifted along until they got hooked. If you have never been addicted, you can have no clear idea what it means to need junk with the addict’s special need. You don’t decide to be an addict. One morning you wake up sick and you’re an addict. (Junky, Prologue, p. xxxviii)

You need a good bedside manner with doctors or you will get nowhere.

Junk turns the user into a plant. Plants do not feel pain since pain has no function in a stationary organism. Junk is a pain killer. A plant has no libido in the human or animal sense. Junk replaces the sex drive. Seeding is the sex of the plant and the function of opium is to delay seeding.
Perhaps the intense discomfort of withdrawal is the transition from plant back to animal, from a painless, sexless, timeless state back to sex and pain and time, from death back to life.

My experience as an addict was very useful to me as writer: the whole syndrome of addiction and withdrawal and the extensions of that and other forms of addiction. It gave me a great deal of material. A writer can profit by something that someone else may not be able to profit from at all. Yet they were very disagreeable experiences. Very boring experiences.

The idea that addiction is somehow a psychological illness is, I think, totally ridiculous. It's as psychological as malaria. It's a matter of exposure. People, generally speaking, will take any intoxicant or any drug that gives them a pleasant effect if it is available to them.