According to leftover game files, Cj can buy drugs from dealers
You have now 3 options to reply to the dealer instead of 2
paying him with actual money
Y:Cj will politely say no and the dealer will leave
N:Cj kicks the dealer in the nuts and he attacks you (Although i want to work more on that... )
After you buy you have 2 ways to smoke differently
Press you or I to smoke in 2 different ways
If you smoke you will get high
Press enter to stop smoking and cj will actually make an animation throwing the cigarette instead of it dissapearing...
Little details count !

EDIT V2 RELEASED
You now have the option to buy a powerfull drug different from cigarettes
The new drug is a pill type of drug
go to any dealer, and press Z
That's it
A drug pill costs 300$ due to how powerful it is
The drug effects take effect slowly so it will be a fun ride!
ENJOY! 

Newest version Released

Fixes:

Game no longer crashes when entering interiors

New Features:

1-You can now properly check how many drugs you have in your inventory anytime, by pressing Tab + I

2-You can now start smoking anytime you want, instead of instantly next to the dealer

3- To buy from the dealer, press any keynumber from 1 to 10. The number indicates the quantity that will be bought
For example, pressing 7 next to the dealer will grant you 7 drugs