It is only shown very briefly, but she actually uses the gelsemium she got from Jane Davis, which she put into his drink. When she leaves the house, there's a short glimpse of the bottle:
As said, she got this from Jane (who imports it from China) for her migraines earlier in that episode:
Claire: I just have a splitting headache.
Jane: Gelsemium. For migraines. I get them all the time. It comes out of China. But don't worry, it's herbal. You have to be careful of the dosing, though. Just two drops.
And in fact Jane herself alludes to how dangerous it can actually be in the wrong doses later in S05E13:
Pardoning or not pardoning your husband, in either case, he's a liability. If it's not treated properly your husband's liver could fail him at any time. It's not the kind of thing I could do. But I could find someone, or I...I don't know if you realize, but even something like gelsemium can be lethal in higher doses.