Boyfriend wants to do drugs

My boyfriend hangs out with people that do a lot of weed. I know this is considered a 'soft' drug but I have made it very clear ever since the beginning of our relationship that I am NOT okay with illegal drugs of any kind, and he said he respected my opinion. Now he's saying that he can't relax or fall asleep and wants to start doing weed to combat this. Apparently when he's with his friends and they offer him a spliff he 'feels he is only saying no because of me' and like I'm getting in the way of his decisions. The way he said that made me feel like drugs were more important to him than me. I've explained so many times why I wouldn't be comfortable if he started smoking weed - I've seen it ruin some of my close friends, it can be very dangerous, it's illegal anyway so what's the point etc. but every time he says he just doesn't get it; weed's a harmless drug. If he's really an insomniac, why can't he just go to the doctors about it and get some thing prescribed legally? Basically, this has all really upset me and made me seriously consider my future with my boyfriend. Am I being too harsh? What should I do?

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