This has been gone over in the Love your liver program with a thorough review by Kelsey who is a scientist. Basically, only take the flush Niacin as Nicotinic Acid and do not take the extended release kind. The niacin is toxin to the liver mantra is pretty much hogwash and was torn apart by Kelsey as being pure nonsense. It's simply propaganda to keep people away from this incredible healing substance.
Unfortunately Ray Peat missed the ball on this one. He missed the 500+ detox pathways that open up literally making the body go into super detox mode. Ray Peat was focused on the wrong mechanisms.“Well, niacin sometimes refers to nicotinic acid or even its slow release form. Nicotinic acid happens to increase your histamine, serotonin, and prostaglandins, all of which are inflammatory. So I don't think it's good for anyone to take nicotinic acid other than as it occurs in food. Niacinamide is a form of that same vitamin that is active in cells and doesn't release those inflammatory problem materials.” Ray Peat