It is highly liver toxic. Medically it's classified as being moderate in it's toxicity. Have you ever looks at liver enzyme values froma patient that has used tylenol for a couple weeks for pain or for fever say at 1,000mg QED? I have, they are most often elevated. Chronic users exhibit similer results to an alcoholic. The biggest threat to Vicodin is not the narcotic/opiat as most would think. It's actually the Aceptometiphen it contains. It is one of the top 10 most dangerous over the counter drugs sold.
Listen, if you need it to get by, use it, but don't be fooled into thinking it is safe like taking a multivitamin or something. It isn't. I guess i've seen the darkside of this drug and ones like it too many times.