How can I persuade my 20 year old son not to smoke dope at home, secretly?

I just want him to get through this phase (I hope it's a phase) without involving us in this illegal activity. I feel that he is treating us like idiots when he denies that he is doing it. He admits to smoking with his friends, and while he knows we don't agree, this pretence at home (plus the smell) is really upsetting me. Any practical tips really appreciated.