His personality has changed...

I don’t know if this is a medical problem or something else, but my boyfriend likes to use recreational drugs, and this is starting to bother me. We were in college together and I have known him for years, but he was never like this. When we began dating two years ago, he was sober, stable, and not interested in partying but as he began working, that changed. The people he began spending time with at work changed his personality and made him more outgoing, but it all came at a price because he is now always interested in socialising and consuming these drugs. I don’t know how to get him to stop because he says it’s impossible to rise in his career if he doesn’t spend time doing what everyone else is doing. What should I do? If this isn’t going to stop, I don’t see our relationship working.
It’s possible that it may take him a while to navigate peer pressure in a corporate environment, but the damage to your relationship will be hard to manage. If he can’t prioritise things in a way that acknowledges your concerns, he will harm your relationship as well as his health. This is also a medical issue because you don’t know what he is consuming, how often, or what kind of effect it is having on him. Ask him to consider speaking to a doctor and find out more about his consumption habits. You won’t have the tools to cope with someone abusing any substance, so if his behaviour doesn’t change, take a call on whether you are willing to put in the work to save this relationship. If it is one-sided, it won’t last beyond a point.

Should I assume that my boyfriend is trying to break up with me based on his rude behaviour towards me? 
If he doesn’t say anything directly, call it out and ask for an apology. If that isn’t forthcoming, ask yourself if you want to be with someone who doesn’t respect you.