My best friend is using drugs what do i do?

i have two best friends one is gay and one is bi they are dating for almost a year now. they both have had strong feeling against drugs and alcohol. then a few months back the gay one started to drink.. because of they people he lived with. then he stopped.. he has been very depressive the whole 2 years ive known him.. now he is drinking all the time and using drugs.. first it was pot.. pot isn't so bad then it was vikes and now perks.. the other day he told me "i didn't know what they were i just took them all" he wasn't in school friday but i finally got ahold of him today making my number private. my other best Friend his boyfriend don't know what to do anymore.. we tried everything.. talking to him he says he doest know what to say. we've tried to bring to him a therapist but he "doesn't do doctors" he says if they give him meds he'll become addicted so i yelled at him saying you are addicted to perks vikes and alcohol i don't know what to do.. can anyone help me please.. and save the comments thank you for those who really want to help!