A drug that causes the user to travel to another world.

[Set Phasers to "Weird".]

Probably published between 1960 and 1978. The hero is testing a drug that causes its users to hallucinate about the same world. If they overdose on the drug they physically transfer to the world. As far as I remember it's a pre industrial world. The hero possibly joins a traveling fair. He builds himself a guitar, an unknown instrument in this world.

 

[Bick]

A wild guess, but does this sound like Piers Anthony's Split Infinity??

 

[Swank]

The Matrix!

 

[sueelleker]

Spellsinger by Alan Dean Foster? He's not actually testing a drug, but is in a marijuana haze when he's summoned to the other world by a wizard. Spellsinger (Spellsinger, book 1) by Alan Dean Foster

 

[Toby Frost]

This sounds like a very Phillip K Dick-type idea, although it's been a long while since I read any of his books. Did these details crop up in The Three Stigmata of Palmer Eldritch?

 

[tachyon]

Sounds like The Dreaming Earth by John Brunner (1963)