If you use a supplement called Big Penis, you should know that it contains an unexpected (or very expected?) ingredient.

Big Penis capsules secretly contain sildenafil, also known by the brand name Viagra, according to the FDA.

“The Food and Drug Administration is advising consumers not to purchase or use Big Penis, a product promoted for sexual enhancement,” the agency wrote in a statement.

Big Penis (a.k.a Big Penis USA) is a “male sexual stimulant,” according to packaging. The FDA operates several International Mail Facilities around the country, monitoring incoming packages for illegal, unapproved, and counterfeit drugs, and found Big Penis at one of the facilities. A laboratory test confirmed that it contained sildenafil, a drug that the FDA regulates.

But the big problem for Big Penis is that the box doesn’t declare that it contains the prescription drug, and that’s a violation of FDA regulations.

Accidentally taking sildenafil, even if you were hoping for Big Penis’ eponymous effects, can be dangerous. The drug can interact with other prescription drugs taken by people with high blood pressure and diabetes, and can dangerously lower blood pressure. And there are other negative effects of taking too much sildenafil: One man’s vision turned permanently red-tinted after a sildenafil overdose.

Big Penis is just one of dozens of sexual enhancement pills marketed as dietary supplements that the FDA has found to contain regulated ingredients. The fact of the matter is, there are lots of issues with the supplement industry and how it’s policed. Supplements often contain prescription drugs or untested and even illegal chemicals, as we’ve reported. A 1994 law puts the onus on the FDA to prove that a supplement is unsafe, as the Atlantic reported, making it difficult to stop illegal supplements before they hit the shelves.

This isn’t Big Penis’ first run-in with authorities, either. Last year, the Australian government’s Therapeutic Goods Administration also found that the tablets contained sildenafil, as well as an antibiotic called chloramphenicoll, both of which require prescriptions in Australia. The TGA teamed up with the Australian Border Force to stop the drug from entering Australia, according to a release.

The FDA keeps a database of “tainted” supplements to keep an eye out for, many of which are sexual enhancement or weight loss supplements. And, according to an FDA statement passed along to Gizmodo, “Consumers should also be on alert for products that offer immediate or quick results and that sound too good to be true.” That’s certainly good advice in general. If you’re thinking about using a dietary supplement, talk to your healthcare practitioner first.