Cannabis is unlikely to be adulterated with other substances compared to other drugs. Although in the past it has happen, in 2008 in Germany 28 people were hospitalised due to cannabis adulterated with lead. The lead was used to increase the weight of the cannabis.
There has also been cases of mould cultures testing positive in cannabis, in places like the Netherlands where cannabis is sold legally.
Although this is not an adulterant you should be aware of synthetic cannabis (see picture). In the past few years Australia has banned synthetic cannabis. Even though synthetic cannabis is made to be functionally similar to THC the effects are different and it can be a lot
tronger. Unlike cannabis synthetic cannabis has caused fatalities.