Aconite

Aconite is the deadliest poison known to man. Merlin asked Gaius for his deadliest poison, Gaius didn't know that Merlin had been turned into an assassin by Morgana. Merlin took a bottle of aconite and poisoned Arthur's lunch, only to discover that Guinevere had already made him lunch. An angry Merlin threw the lunch in the stables where it was eaten by a pig, who then died. This was discovered by Gaius, causing him to suspect that something had happened to Merlin. (A Servant of Two Masters)