Get them to do something dumb under the influence and you'll get another promotion when they're fired.

I hope drugs isn't the idea of your promotion treat.

The best advice I've ever heard was to remember that a work drinks function is WORK. Limit yourself to two alcoholic drinks total, one over the first ninety minutes, one only thereafter. If they take ecstasy, great. Be right among them while they realise how intensely they love you. Don't show your disapproval. If they offer you one, thank them, put it in your pocket for later, then when not observed, throw it in the rubbish. Your professional behaviour IS observed.

And what exactly is bad about this?

It's no worse than getting drunk, honestly. If nothing else, at least don't alienate them by being judgemental about it.

well you've been put into a tough spot. Do you keep quiet to try and make your job easier but possibly get in trouble for knowing that they were planning on doing drugs at a work function, or do you report them and make sure you keep your job but also make your job much harder if they dont get fired and hate you. Your going to have to decide which repercussions you would rather live with.

It's ecstasy not heroin, calm down guys

Just don't take any, if you think it's effecting their work and you can do a random drug test, recommend it to whoever is above you. Otherwise leave it as is. Taking drugs when not at work might not be something you agree with but if it doesn't affect you then let people do what they want to do.