Last night I was reading about feline nutrition and came across a statement -- "dl-alpha lipoic acid is highly toxic to cats". My jaw dropped open. I reread it 5 times, then looked for any references which it didn't give because this was common knowledge, I guess. So, I looked it up this morning. And yes, indeed, it is toxic. At distressingly low levels -- 30 mg/kg. Dante weighs about 11 lbs or 5 kgs, so about 150 mg. Maybe less. I was using 150 mg capsule, divided up for 2-4 days, so at the most he was getting 75 mg, which is under the tested level, but maybe not under the effective level. (The researchers tested at 0, 30, and 60 mg/kg levels.) There was hepatocellular toxicity at the 30 mg/kg level, but maybe lower? Until someone comes back and tries 10 mg/kg or 20 mg/kg, there will be the question of where the toxicity level lies.
The good news is it effects the liver, which can and does regenerate. The other 'good' news is I gave it to him off and on, but mainly off because I never could remember to do it. So, my laziness and forgetfulness may have saved him.
For a summary of the research, see Lipoic acid toxicity research