Why not write him a letter saying how you feel.
Make it clear that you love him and as your son you always will but that the way he's being right now and the effect he's having on the family makes it very hard for you all right now, give some examples.
Say that you're there to help him if he needs it, that you will understand that it may be tough at first but that as he's an adult now if he won't agree to help himself and act reasonably in your house then he needs to find somewhere else to live.
You can compromise with him, that in return for reigning himself in, small steps such as giving up the drugs, drinking sensibly only and unreasonable behaviour that he can stay, that you will trust him (stress trust has to be earned) with relaxed curfews and be easier on him and his girlfriend such as inviting her round for meals and such and letting her stay over sometimes.
Maybe give him an incentive such as if he saves a certain amount of money by a certain date (ie incentive not to spend it on drugs and alcohol) that you will match it so he can do something special such as a holiday for him and his girlfriend.
Also explain why the drugs scare you so much, show you've done some research and are not just a nagging mum and put some leaflets in he can read. Say you're worried sick what he could get himself into and you don't want to see him suffer or worse case scenario lose him
Try here for some more info...
http://www.talktofrank.com/home_html.aspx
Hope everything works out xxx