The leaves and twigs of the yerba mate plant are dried, typically over a fire, and steeped in hot water to make an herbal tea. Yerba mate may be served cold or hot. This beverage, commonly known simply as mate, is popular in parts of South America. Like black tea, yerba mate contains caffeine, which is a stimulant.
In the U.S., yerba mate is widely available in health food stores and online. Proponents of yerba mate say that it can relieve fatigue, promote weight loss, ease depression, and help treat headaches and various other conditions. There's no definitive evidence that these claims are valid.
Yerba mate isn't likely to pose a risk for healthy adults who occasionally drink it. However, some studies indicate that people who drink large amounts of yerba mate over prolonged periods may be at increased risk of some types of cancer, such as cancer of the mouth, esophagus and lungs. Drinking very hot yerba mate — 149 F (65 C) or hotter — is associated with a higher risk of cancer than is drinking yerba mate served at cooler temperatures.
One possible explanation is that yerba mate contains polycyclic aromatic hydrocarbons (PAHs), which are known to be carcinogenic. (Tobacco smoke and grilled meat also contain PAHs.) More investigation needs to be done into the safety and side effects of yerba mate.
If yerba mate is your cup of tea, enjoy it in moderation. But, as always, check with your doctor before trying any herbal product.