Vitamin E. Cats can't overdose on it, and the only side effect of HUGE quantities is loss of appetite (so it's a good treat for overweight cats). I found out about it while trying to use it as a wound salve, but when Smudge found it so tasty I looked around for articles about it and learned that it's a very safe treat, and particularly good for cats who eat a lot of fish (particularly tuna).

Love, Columbine