I went through this with my first fiancé. He was clean for 3 years during our engagement. He'd then slipped up and had started to do cocaine and other drugs (he was a psych nurse). All of the lying, the mean dispositions, etc. It was a disaster! He only got worse.
Luckily, I knew enough to listen to my intuition. He was, prior to this, the nicest guy anywhere. I first postponed the wedding and as he kept spiraling downhill, and the drugs were changing his personality, I then broke off the engagement. Our lives were very intertwined by that time. It was a nightmare. I had to have the police remove him from the house and the court restrained him. He had gone over the edge and was threatening my life, repeatedly. He'd turned into a very scary character, someone I did not recognize.
Please be careful. Addiction/drug abuse can change people into someone you do not recognize.

WC