He took what he had on-hand - I don’t believe we know what kind of poison it was specifically, as he’s mentioned to have ‘fortified’ himself against several different kinds. He eventually had one of his bodyguards kill him when the poison didn’t work. Poison is considered an ‘easier’ way to go than being run through with a sword, so he was probably hoping for a bit of ‘the dosage makes the poison’ kind of thing.