Cannabis plants produce a second important substance called cannabidiol, or CBD. The compound appears to reduce the risk of psychotic disorders, by counteracting the effects of THC. But CBD and THC are made from the same precursor chemical, so maximising THC reduces CBD levels. This may have important implications. High-THC cannabis, such as skunk, appears to cause more mental health problems than low-THC cannabis, but the reason may be the lack of CBD to balance out the THC. A US study published this month found that as cannabis potency rose from 1995-2014, the ratio of THC to CBD rocketed from 14:1 to 80:1. Englund demonstrated the power of CBD in a striking study in 2014. He gave THC to volunteers and noticed they soon displayed acute psychotic symptoms. But when he gave them CBD too, they were largely protected from THC’s effects.