Thousands of people have experienced disturbances in taste after eating pine nuts, but the cause remains unknown.

A bitter metallic taste that develops within two days of eating pine nuts and lasts up to two weeks was first reported in 2000. In 2009, a possible link between “pine mouth” and the consumption of the nuts of Pinus Armandii, a species of pine not traditionally eaten by humans, was reported by the French Food Safety Administration.

New research into the fatty acid levels of different pine nuts has found that most pine nuts sold in the United States are a combination of nuts from different species including Pinus Armandii. However, which pine nuts cause pine mouth still cannot be definitively predicted.