You should think twice or thrice before buying any dietary supplement online because some of them have been found to be tainted with undeclared prescription drugs and potentially harmful substances.

Consumer Reports, an American nonprofit consumer organization dedicated to unbiased product testing, investigative journalism, consumer-oriented research, public education, and consumer advocacy, has warned about potentially dangerous substances found in some supplements sold online.

Some products have been found to be tainted with sildenafil (Viagra), tadalafil (Cialis), and fluoxetine (Prozac), which are prescription-only drugs. They should be used only under the supervision of a doctor.

The U.S. Food and Drug Administration (FDA) found these prescription drugs in some weight-loss and sexual-enhancement supplements that were sold on Amazon and eBay.

The tainted products on Amazon were sold as Number-One-Best Seller or Amazon’s Choice, according to the FDA.

It is illegal to sell dietary supplements containing undeclared prescription drugs. The FDA says manufacturers should accurately mention the ingredients on the label. However, the agency found “hidden” drugs in many of these dietary supplements.

In a statement, Amazon said it was “reviewing the information from the FDA” and would take action accordingly, while eBay told Consumer Reports it was “reviewing the site” and would be removing the tainted products.

If you take any dietary supplement, make sure you check the FDA’s database on tainted products. Even if the supplement you want to take is not on the FDA’s list, you must still be cautious and seek medical advice before using one.

“Talk with a doctor or another health care professional about whether the ingredients may interact with any medications or supplements you’re currently taking,” according to Consumer Reports. The FDA advises people to immediately see their doctor if they experience any side effects from using a supplement.