Question

Say I used Toxic on my opponent. Then my opponent used Toxic back. BUT what if my pokemon has the Synchronize ability, will the toxic poison from my toxic be replaced with regular poison? Or will nothing happen?

Answer 1

Sychronize would fail to take effect, because the opponent already has a status ailment.

Answer 2

This has been asked before. :)

http://pokemondb.net/pokebase/4049/does-synchronize-pass-on-toxic-poison

Comments

I'm saying like Umbreon uses Toxic on Swampert, then Swampert uses Toxic on Umbreon.  Is Swampert badly poisoned because of Toxic, or normally poisoned because of Synchronize?

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You already have a status, so it would fail.