Yerba mate is not likely to pose dangers for adults who drink it in moderation and are otherwise healthy, according to the Mayo Clinic. A few studies, however, have shown increased cancer risks in people who consume large amounts of yerba mate over a long period of time.
Some studies have shown that the consumption of yerba mate in large quantities over a prolonged period may increase risks of cancer of the mouth, esophagus and lungs, reports the Mayo Clinic. This seems to be especially true for people who also use tobacco or alcohol, and nutritional deficiencies and poor oral hygiene may also play a role.
Due to its smoked drying method, yerba mate contains small amounts of polycyclic aromatic hydrocarbons, known as PAHs, which are carcinogenic, notes the Mayo Clinic. PAHs are also found in tobacco smoke and grilled meat. Yerba mate company Guayaki claims that it has worked to reduce PAH levels by switching to an air-drying method for some of its products, including at least one of its yerba mate products.
The Mayo Clinic advises checking with a doctor before trying any new herbal supplement, including yerba mate. For most healthy adults, however, yerba mate can be regarded as safe to drink in moderate amounts.