Toxic Blood is a mutation in the Blood and Wine expansion, which allows adding abilities from the Alchemy tree to the additional slots.

Description

• Each time you are injured in melee combat, the attacking opponent receives damage in the amount of 1.5% of damage dealt for every point of your Toxicity level (currently 1.5%, to a maximum of 150%)
  • As of Patch 4.0 it's 3%

Tips

• The description can be misleading as the 150% limit applies only to the base toxicity pool, thus using effects that increase the pool capacity will also increase the limit.
  • Using skills like Acquired Tolerance, Metabolic Control, and wearing Manticore School Gear can increase the limit to nearly 500%.
• Like most "punishment" effects, the damage returned depends on the damage Geralt receives. Which means that when running a tank build the effect will be minute or if the damage is negated by using Quen, the returned damage will also be 0.
  • Effectively even if the toxicity pool is increased to the absolute limits, the damage will be very low.
• The effect will cause Geralt's blood to turn into green acid, which apart from the damage, will also cause the enemy to back away a bit when it hits, similar to how alghoul spikes affect Geralt when they are out.