Red mutagen is an alchemy ingredient and is used to craft Greater red mutagens. They can also be used to augment abilities, but are not as effective as their greater counterparts. When placed in the diamond-shaped slot in the character panel, they give +7% to attack power, which increases by 7% up to a max of +28% if all 3 skills attached to it are red.
Blood and Wine expansion
With Blood and Wine, the player can now craft these with other ingredients that don't include Lesser red mutagens.