Of course, many other drugs can be fatal if you go way overboard. Even Epsom salts, which are commonly used as a laxative, can cause cardiac arrest. A hospital in Scotland reported a case of attempted suicide by Epsom salts in 2009. The woman ingested an incredible 4.4 pounds of the stuff. She suffered some acute cardiac complications, but doctors were able to save her.
It's not easy to determine the fatal dose of over-the-counter drugs. Take the example of acetaminophen. A person's ability to handle the drug depends on a variety of factors, including the condition of their liver, how much they've eaten, and whether they take the pill in conjunction with alcohol. (Bad idea.) Compared to other over-the-counter drugs, acetaminophen has a relatively narrow safety margin—that is, the difference between a safe-but-effective dose and an overdose is relatively small. Doctors have reported liver failure from as little as 2.5 grams in a day, which is 1.5 grams less than the approved limit.
The FDA has considered reducing the approved daily dose (PDF) of over-the-counter acetaminophen, but that wouldn't help in all cases. Many patients don't realize how much of the drug they're taking. Those who are prescribed the painkiller Percocet, for example, might take over-the-counter acetaminophen as an adjunct for different or breakthrough pain. The problem is that Percoset contains acetaminophen, and the combination can easily put them over the dose limit.http://www.slate.com/articles/health_and_science/explainer/2011/12/emergency_contraceptives_over_the_counter_are_they_more_dangerous_than_other_drugs_.html?