Mutagens grant significantly better combat bonuses than Elixirs, but come with negative side effects. They also take longer to have an effect - after drinking one, a creature is Sickened 3 until the start of their next turn, at which point the effects of the Mutagen kick in. Each dose of Mutagen is crafted alongside its own antidote, which immediately ends the Mutagen’s effects when imbibed as a 1 Major Action (Equipment, Manipulate) ability.
Anyone who drinks a Mutagen that is crafted at a higher Tier than their personal Tier is Sickened 3 for 1 hour, and does not gain any benefits from the Mutagen. Any reference to “per Tier” in a Mutagen’s description refers to the Tier of the Mutagen, not the person who drank the Mutagen.