How do I use mutagens?

Mutagens boost your abilities which allow a mutagen slot.

You can loot one from a defeated enemy or you can create one as a byproduct of alchemy.

To apply a mutagen, go into quick menu and choose meditation (Default key: CTRL), then select the Character option and find an ability that has a mutagen slot (list of abilities with mutagen slots). Select the ability, and choose Mutate (Enter). Select the desired mutagen from the list and choose Mutate Ability (Space).

The mutation cannot be reversed.

Mutagens usually take one of three forms: lesser, basic and greater. Each of these forms raises the character’s statistics, but by a different amount:

• Critical effects mutagen: Increases the chance of causing critical effect.
• Range mutagen: Increases the range of the Aard, Igni and Axii Signs.
• Vitality mutag: Increases vitality.
• Power mutagen: Increases sword damage.
• Strength mutagen: Increases damage reduction.
• Concentration mutagen: Increases Sign damage.
• Madness mutagen: +2 armor, +5 vitality
• Enhancement mutagen: Increases adrenaline generation on hitting foe

You can only use them in combination with certain abilities in your skill tree - e.g. if you activate "fortitude" in the basic branch, a small circle allows you to use a mutagen of similar effect. Definitely something that should have been explained in more detail.