Looking over the list of drugs in SR4 (p. 244), it doesn't seem to say there is anything keeping drugs from stacking. In that case could you take Nitro and Kamikaze, end up with +4 str and High Pain Tolerance 9? or Cram and Jazz for +2 IP? If so, that could make for a pretty interesting character or opponent. With all of the major combat drugs active though, you'd suffer 21S unresisted, that'd kill just about anybody. Still, an interesting idea.

I think this was talked about a little while ago, when someone was asking about the prices, but ya the general gist of the whole thing I got was that it does stack up, but to the users own risk.

In general the effects of drugs should stack - but combining different drugs can lead to undesired side-effects as well...

The GM who has her players do this often should do some studying on the effects of combining narcotics, and hit the PCs with those.

For most NPCs, they won't be around for long enough to care anyway...

Here's a quick link on the real-world issues involved in mixing drugs:
It's All in the Mix
Note that I'm no expert, but I think translations of SR drugs to real-world drugs would go something like:
Bliss->Heroin
Cram->Possibly Speed or Ecstasy, not really sure.
Jazz->Speed
Novacoke->Cocaine
Zen->Acid?

If any players mixed drugs in my games then I'd certainly be tempted to, as a general rule, multiply the side-effects of each by the number of drugs they had taken. Nitro with its 9S damage code is kindof as good as you can get mix-wise and was probably created by someone with a degree in biochemistry or equivalent. It probably includes small quantities of drugs to conteract and stabilise the body's natural functions, and note that Nitro's Novacoke+Narcotics mix sounds fairly similar to the 'Speedball' mentioned in the above article.