Frankly cannabis is a good mixer. There are no major health risks which is probably why many recreational drug users smoke it.

Please note: there have been very few scientific studies into the effects of combining psychoactive drugs. The information presented here is anecdotal. It is based on the subjective reports of experienced users. Different people will respond differently to different drugs and drug combination. The effects may be different or dangerous for you. Know your body.

alcohol makes both drugs stronger; manageable at low doses, but can cause vomiting, dizziness, and increased paranoia with large amounts of both

amphetamines can take the edge off the harsh speed buzz

cocaine unusual combination (uppers and downers) but can help modulate the grinding intensity of coke

ecstasy extends and expands the E experience; used to help bring on the high, it mellows the intense rushes and helps with the come-down; makes the ecstasy more psychedelic.

heroin complimentary effects

ketamine no reported problems, but make sure you stub it out before taking K; K amplifies certain cannabis effects (especially closed eye visuals and mind tunnels)

LSD dulls the experience in the come-up; heightens the peak; brings back the effect during comedown

mushrooms as above; can increase weirdness of the experience

tobacco is a nasty drug