Paracetamol/acetaminophen(Tylenol) is a "helper drug". It works with other drugs, in this case codeine, to allow it to work more efficiently. That's why they bundle them together.
I'm afraid you're mistaken, a "helper drug" would be more like pairing caffeine with aspirin/paracetamol. It helps the analgesics work more effectively. Excedrin (US) and Anadin Extra (UK) both contain these three compounds
In the case of Codeine+Paracetamol, Paracetamol is included in the formulation to help curb codeine addiction. You'll sooner go into liver failure because of an overdose of acetaminophen than become a codeine addict.
Yes. The problem with Paracetamol is that it depletes an antioxidant in the liver and if you don't have enough there goes the liver. Some people can't handle it at all, not in an allergic way but in an unable to process it way.
Also in its metabolism, Paracetamol generates Reactive Oxygen Species (like hydrogen peroxide), which causes oxidative stress that antioxidants would normally help control...so it's sort of like a double whammy.
A triple whammy comes in when you take ethanol and paracetamol together, as the enzyme that is implicated in paracetamol's hepatotoxicity is induced by ethanol--generating more ROS's and depleting more antioxidants, which can lead to liver failure.
This is why even in the worst hangover I won't take any Tylenol.
<Sorry if i'm being too science-nerdy...For several years my research focus was on drug metabolism, specifically the
enzyme that metabolizes paracetamol...>