In Let's Kill Hitler, an episode from the 11th Doctor, the Doctor and his companions encounter the usual assortment of difficulties and obstacles.
However, the biggest difficulty contains something that I don't quite understand.
When Melody Pond kisses the Doctor, she doses him with a poison via the kiss. Apparently this poison somehow "prevents regeneration".
How is this possible? Is there any precedent for this being achievable in other episodes (including prior to the series reboot)?