Nutritional supplements can be overkill if taking a multivitamin and can also cause allergic reactions and interact with medications, says a U.S. expert.

“Most people think that if they get a substance over the counter, it’s all right to take,” says Rudy Nelson Lea Jr., a family medicine physician on the medical staff at Baylor Medical Center in Garland, Texas. “But if you’re taking prescription medicine at the same time, it’s often not all right.”

Supplements can cause allergic reactions and interact with medications, so consult with a doctor and pharmacist before taking them, Lea advises.

For example, echinacea is used to treat respiratory infections such as the common cold. “Proponents suggest that it increases the response of white blood cells — the cells in the body that fight off infections,” Lea says. “However, people who are on medication that suppresses the immune system should avoid echinacea.”

Iron supplements are used to treat low blood iron or anemia by helping the body make red blood cells, but iron may affect the results of certain medical tests and also cause bloody diarrhea, constipation and fever, according to Lea.