- If I attack with Sceptile's "Power Poison" attack, do I have to discard an Energy card if I don't want to Poison the opponent's Pokemon?
- It is not optional. Even if the damage would be enough to Knock Out the Pokemon, you still have to discard an Energy and Poison it.
Source:
Primal Clash FAQ (2015-02-05), TPCi Rules Team (2015-02-05)