Have you ever confronted someone with a drug problem. ? How do they react?

I've been with my partner for a few months short of a year. I think he started abusing prescription drugs in April. He doesn't get super fucked up around me, just enough to know somethings off.. I've mentioned it a few times, he's taken a drug test it was neg. But it could've been wrong. Or the substance he uses wasn't on there.

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I wrote him a letter and was not mean or anything. I tried to show I love and have concern for him. But I did set boundaries I told him hes not allowed at the apartment while I'm not here and I don't want to be around him when he's high..until he can prove he's sober. Which I feel is fair.

Well..he obviously got mad. I told him if he wants to buy a hair drug test to prove his innocence then he can, refuses to buy the test himself because I'm the one accusing him. But I don't trust how accurate pee tests are. He's angry and keeps trying to argue. He's also tried to guilt trip me twice. He's not using logic to talk about stuff.

I'm just wondering if this is a typical reaction from someone who's guilty? Or if he's reacting this way because he's hurt by me?

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I'm almost certain he's on drugs but I have no proof other than him acting high around me. He's always broke. Few weeks ago I saw him take $100 out of the safe and I asked what it was for and he said "uhh"...."well my checks have been hitting later than usual". But that doesn't explain what the 100 was for?. He was on the way to his grandma's to mow, she was also paying him 100. He also sniffs a lot like someone who does coke would sniff, he also has a lot of nose drip (if you know about that then you know what it sounds like). He acts tweaked out every now an then. But most times it seems like he's using downers, hell say stuff that doesn't make sense, voice is deeper, eyes are red/glassy and heavy. He usually acts like this after a bathroom trip and it only lasts like 20 min. Seems more anxious and uptight, doesn't take a lot to make him mad. So I know It's not weed..also wouldn't care if it was weed.