I'm thinking about donating Plasma, but I've recently smoked weed (I do so occasionally) and I was curious if that would make me ineligible.

Most people I know that donate blood and/or plasma are drug users.
One was honest and told them he did some drugs and they wouldn't take his blood because he told them that.

Over here, they prefer to take plasma from people who are established blood donors. It may be different where you live.

Anyway, the general rule is to tell them that you have taken and when. It's surprising how many drugs and medications you can take as long as they know and there is a reasonable period. For instance, if you've taken asprin, they normally ask you to wait five days before donating. If you are on hay fever medication, you can give blood, but you must tell them the exact type.

If and when they ask, if you tell them you have used, they will not accept the blood.

Sadly in the inner city drug users are one of the main sourses of blood.