Despite widespread consumption of yerba mate (particularly in South America), clinical research examining its health effects remains limited, and the overall evidence base is weak. Many studies could be prone to bias due to the lack of a control group, small sample sizes, being industry-funded, and being of poor methodological quality in general.
Consumption of yerba mate tea is associated with an increased risk of cancers of the upper digestive tract (i.e., the mouth, esophagus, larynx, and pharynx), most notably when consuming more than one liter (about 4 cups) of yerba mate daily.[1][2] Associations between yerba mate consumption and bladder and lung cancers have also been reported.[3][4] This could be due to the presence of carcinogenic compounds (e.g., polycyclic aromatic hydrocarbons) produced during the processing of yerba mate leaves, as well as the thermal damage caused to the upper digestive tract when consuming yerba mate hot.[1][5]
In general, yerba mate is usually tolerated well. Side effects are uncommon, but nausea, insomnia, and irritation of the lining of the mouth or stomach have been reported.[6]