He's My Bff, Closest Friend I've Ever Had But Am I Threatening It?

My best friend and I are SUPER close, I don't use the term best friend lightly; he truly is, and we're closer than most people are to their 'best friends'.

Unfortunately, after two years of enjoying this relationship, due to mephedrone (legal drug) and alcohol; we had sex... and it seems to becoming frequent. I don't want to risk my friendship with him: I hope it's not going to ruin it. Hopefully if we keep it as a sex thing we won't... right? :/

Only time will tell...