Need help on Roommate who is using Drugs?

Don't enable him at all. 

This "loaned" money is lost. You may as well kiss it goodbye. Even if he paid some of it back in the past, he has since acquired additional funds from you.

I know you say he may attack you, but that's an unacceptable situation. Better to not report him for drug use, as he actually may need to self medicate. 

What you should do is request immediate rehousing for one of you, on the basis of safety. Make sure the person you contact is aware of your heightened concerns of physical harm.

You absolutely must keep yourself safe. Do something about this situation before it's too late.

If this is a legitimate situation, he could and might attack you regardless of what you do.  Does he have any family that you know or have their phone number, call them.  Also STOP giving him money, you are not only enabling him to be an addict, you are digging your self deeper and deeper in to this situation and just making things worse for yourself.   You could also consider moving yourself.

So you are enabling his behaviour.......... stop lending money and report him TODAY

I say mind your own business. Don’t rat him out, but don’t help him either. Tell him you can no longer give him money. If he attacks you, then you have every right to press charges or kick his âss. Don’t allow people to bully you. Guys like that sense your weakness and exploit it. It’s time to stick up for yourself. If you’re really that afraid of him, request a new roommate. But don’t let him take advantage of you. If he’s really behaving that badly, they’ll figure out he’s high on their own soon enough. You don’t need to get involved. Just don’t let him see that you’re afraid. Just man up and say no to enabling him. If he assaults you, feel free to report it to whoever you want. His drug use isn’t your problem to deal with. I’m not a big fan of snitching and tattling. If he hurts you, that is your problem, and you have a right to report any crime which you are a victim of.