You’re in luck! It's cannabis.
The man has an unusual medical condition that causes plants to grow from his skin. Instead of curing him of this condition, the doctor points out that the plants are marijuana plants, which would be good either because he could sell what he grows, or he could smoke it and enjoy using drugs. A doctor that sees these upsides of his condition (rather than curing them) comes across as humorously untrustworthy.