There are a couple of distinct adulteration issues, AFAICT. I’m pretty sure the most common one, for all classes of drugs, is cutting with inert filler. For heroin and cocaine this is actually pretty nasty, since the popular inert substances to cut with are really nasty to stick in your nose and veins. But marijuana “fill” is usually just inert plant matter which, at the worst, tastes awful and gives people headaches.

There are rare circumstances of “enhanced” marijuana, occasionally with PCP or cocaine (marijuana laced with acid is a common myth, but would be absolutely pointless, since LSD decomposes at a lower temperature than the vaporization point of THC). This is pretty uncommon, not because drug dealers are scrupulous but because it’s lousy business practice: while “superweed” might gain a loyal clientele, it would alienate at least as many as it attracts, and those additives are pretty pricey. If you have cocaine, it’s better for the bottom line to sell it as cocaine than to use it to flavor weed.

There are authentic, dangerous adulterants for marijuana, but I don’t see them mentioned here: namely, results of unsafe agricultural practice. Toxic insecticides are moderately common; heavy metals are rare but not unheard-of (although tobacco is far worse as a heavy metal magnet). AFAIK, insecticides are the only authentically dangerous adulterants which show up much in marijuana (other drugs are a different story, of course — plant matter is pretty trustworthy in that any filler people choose is likely to be mostly cellulose; when people are cutting, say, a white powder, they come up with some awfully stupid things to ingest).