The ingredients in Tylenol PM, acetaminophen and diphenhydramine, have different lethal doses. The lethal dose of acetaminophen in rats is 2,400 milligrams per kilogram, reports the Food and Drug Administration. The lethal dose of diphenhydramine is between 50 to 500 milligrams per kilogram, according to the National Institutes of Health.
Tylenol PM contains 500 milligrams of acetaminophen and 25 milligrams of diphenhydramine hydrochloride per caplet, as stated on Tylenol's website. Acetaminophen at lethal doses causes liver toxicity, liver failure and death. At lethal doses, diphenhydramine toxicity causes facial flushing, hallucinations and death. The number of tablets that cause an overdose varies depending on the weight of the person taking the medication.
Permanent damage from the acetaminophen in Tylenol PM and other medications containing acetaminophen can happen even when the toxic amount is ingested over a short period of time. The damage done by acetaminophen can happen over time, causing irreversible and potentially fatal liver damage, according to research documented by the Huffington Post.