Okay...I am writing a script, and I need a poison.

This guy is going to do something that he is really nervous and excited about. It is dangerous, to some extent. There is someone he trusts, who pretends to be on his side, but actually wants him to die.

She gives him something to "calm his nerves" but actually turns out to be poison. Not necessarily strong enough to kill him right away.

I'm thinking she could give it to him and say that it is prescription pain killers of some kind? But what could it actually be? He probably does not do a lot of drugs, so it's okay if it were pretty obvious that it is not what she says it is.

I'm looking for something that could kill him, but would more likely make him really sick.