How much acetaminophen is toxic? The word “toxic” is a bit misleading because acetaminophen toxicity comes in two varieties. There is the situation where a large dose is taken in a short period (24 hours or less), called acute toxicity or overdose, and then there is a chronic form that happens over months or years. Either causes liver damage.
The acute form of understanding how much acetaminophen is toxic takes place when too much of the drug is ingested for the liver to safely get rid of. But to know how much acetaminophen is toxic, the actual amount will depend on the person, their body weight (more liver is better) and what else they may have taken – for instance, alcohol increases the risk. The package recommendation is set at a safe level for most people of 4 grams a day. That can mean eight, 500mg Tylenol extra-strength or it can mean any combination of products that contain acetaminophen. Acute toxicity often happens when people unknowingly combine products (headache, flu or cold medicines) that each contain the drug. It’s the total that matters, not the name on the label.
How much acetaminophen is toxic? The LD50 (Lethal Dose 50%) for acetaminophen varies by species. LD50 is a statistical measure of how much of a drug it takes to kill half the population who takes that amount. For acetaminophen, the number given is 338 mg/kg. On the surface, that means a 75kg person (165lb) would have a 50% chance of death if they took 25 grams of the drug.
But there are two problems with LD50. The first is that we don’t test for a number in humans. Mice may or may not be a good model. Experience with actual overdoses show that liver failure can happen with as little as a 6 gram dose. So, somewhere between 6 grams and 25 grams would be one answer. But there’s another difficulty. Overdose with acetaminophen doesn’t kill right away. In fact, the initial symptoms may be quite mild and pass for a day or two until jaundice develops. By the time that happens, the actual amount of acetaminophen in the body may have fallen and no accurate measure can then be taken.
Considering these reasons in how much acetaminophen is toxic (as well as the variability in the population) is what gives us the 4 grams a day rule. For chronic exposure, it’s even harder to come up with an exact figure. Some patients take a great deal of acetaminophen for a considerable time without showing ill effects. They may be getting some liver damage, but not enough to detect.
Currently, because acetaminophen toxicity is the most common type of drug related overdose in the US, scientists are taking a careful look at what level may be safe. One study tracked a patient who was taking 5 to 6 grams a day from multiple medications over 6 to 8 weeks. This person developed toxic hepatitis without having any other risk factors.
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