They’re explaining how him smoking weed led to his condition worsening.
“But it’s just herbal!” the patient protested. “How can it be bad?”
Dr Jenkins sighed. “Apricot stones contain lethal amounts of cyanide. There is a certain plant in my back garden – if you sit under it for just ten minutes, you will die. Just because it’s natural doesn’t mean it’s safe for you!”
The man seemed to accept that, and after he and the doctors parted ways, Dr Smith asked, “What is that plant that kills you if you sit under it?”
“A water lily.”