Venom Coating

Whenever you make an attack that deals poison damage, you can use your bonus action to coat weapon or ammunition of your choice. When next you deal damage with the coated weapon, have the target make a Constitution Saving Throw. It must pass a DC of 16. On a fail, the creature takes an extra 2d4 poison damage.

The coating only lasts for 1 minute, and cannot be stacked. If the weapon is a Flame tongue weapon, and the fire is active, this poison is immediately burnt out.

You may use this feat if you have the ability to breath poison damage. i.e. A Dragonborn breath weapon in poison form.