For people with a peanut allergy, peanut butter can cause reactions, including anaphylactic shock, which has led to its being banned in some schools.
The peanut plant is susceptible to the mold Aspergillus flavus which produces a carcinogenic substance called aflatoxin.
Since it is impossible to completely remove every instance of aflatoxins, contamination of peanuts and peanut butter is monitored in many countries to ensure safe levels of this carcinogen. In 1990, a study showed that average American peanut butter contained an average of 5.7 parts per billion of aflatoxins, per the U.S. Food and Drug Administration guidelines of 20 parts per billion.
Some brands of peanut butter may contain a small amount of added partially hydrogenated vegetable oils, which are high in trans fatty acids, thought to be a cause of atherosclerosis, coronary heart disease, and stroke; these oils are added to prevent the peanut oil from separating. Natural peanut butter and peanuts do not contain partially hydrogenated oils. A U.S. Agricultural Research Service (ARS) survey of commercial peanut butters in the U.S. showed the presence of trans fat, but at very low levels.At least one study has found that peanut oil caused relatively heavy clogging of arteries. Robert Wissler of the University of Chicago reported that diets high in peanut oil, when combined with cholesterol intake, clogged the arteries of Rhesus monkeys more than butterfat.Peanut butter can harbor Salmonella and cause salmonellosis, as in the Salmonella outbreak in the United States in 2007. In 2009, due to mishandling and apparent criminal negligence at a single Peanut Corporation of America factory in Blakely, Georgia, Salmonella was found in 46 states in peanut-butter-based products such as crackers, peanut-butter cookies, and dog treats. It has claimed at least nine human lives as of 17 March 2009 and made at least 691 people sick in the United States.