At first the questioner asked Can you use Uncanny Dodge against extra damage effects?, the first answer does not answer the question to the questioners full satisfaction, so he asked for a specific case (which he should have included in the question, but is passively included by asking for "extra damage effects").

Another user and the answerer mentioned, that he should ask this question with the additional details in a different question, which he did by asking this question: Does Uncanny Dodge halve poison damage?

But that question got closed as a duplicate because it is really the same question.

Luckily there was a user which could post a useful answer including the specific case mentioned by the questioner.

I suggest merging the two questions, because they are about the same issue, but some users might not agree on the definition of extra damage effects.