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. Smoking in combination with yerba mate seems to greatly increase the cancer risk.
One possible explanation is that 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 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. My favorite: Guayaki Chocolate.