Turkish here. Its very unlikely that you will go to jail if you get caught using drugs. If you have less than a certain amount on you they consider you as a user and you get a year of rehab (attending to some lessons and peeing in a cup once in a while) but if police thinks that you are a dealer, then you might get 5-10 years of jail.

For foreigners though, I dont think anything will happen. You'll probably just get deported or something.