"This blend of 2 different herbs removes toxins, plus, it reduces damage taken and guards against poison for a short period."
— Inventory description
"2種のハーブを組み合わせたもの。 解毒作用がある。 また、使用後一定時間、ダメージの軽減と、 防毒の効果がある。"
— Inventory description - Japanese
The (R+B) mix removes any toxin effect, reduces all damage taken and makes the player invulnerable to poison effects for a short time.
It's possible to see how much time is left by looking at the symbol at the UI at the bottom right of the screen, next to the bullet count. The symbol only appears when the (R+B) effect is active.