That's not necessarily why you take drugs.

Uppers like cocaine (which he was very fond of at the time) are often consumed to be more productive or to "enhance" an otherwise already positive time. From what I've heard, he was introduced to coke at a party, which makes perfect sense. Cocaine abuse starts with making a party exhilarating, then it continues with giving you so much energy while working, finally you simply take the stuff because your brain doesn't know how to work with natural levels or serotonin/dopamine/noradrenaline anymore.

To name another class of drugs, taking psychedelics (which I haven't heard of King having much business with) to hide from your inner demons would be a profoundly bad move since those tend to forcibly confront you with yourself. Great if you have the feeling that deep down there's something you need to address but you don't know what. Terrible if you know there's something but you can't handle dealing with it.

Downers like cannabis (which King apparently did at least occasionally use) can be used to silence the bad thoughts. Putting up a smokescreen between your conscious and your subconscious isn't exactly the best way of handling things but at least you're not pouring fuel onto the fire.

Drugs of all kinds can still take your mind in directions you normally would've shied away from, that's true. And a sustained drug habit of any kind is often indicative of an underlying problem; happy people don't mess with their brain chemistry as often. The specific underlying problem can vary wildly – unassisted psychological distress, physiological issues like chronic pain, performance anxiety, peer pressure... the list goes on and on.

Of course, given that King also had a sustained alcohol problem and apparently at one point abused everything he could get his hands on, he definitely seems like someone who couldn't handle what was going on inside of his head. Thankfully he had the support he needed to overcome his drug problem.