What kind of poison causes siezures very quickly?

TRUST ME, I AM NOT A MURDERER OR ANYTHING LIKE THAT. Simply curious. I am writing a novel, and one of the characters is poisoned by her ex-lover. She is paralysed(from being stabbed in the spine), then he forces her to drink/ingest poison. The effect only takes around 15-30 seconds for her to realize something is wrong and the siezures to begin. If you could help, I would be very grateful. Thanks for reading and answering.