The FDA just warned consumers to stop using certain Zicam Cold Remedy products designed to be used in the nose. The agency said it has received more than 130 reports of loss of smell associated with the use of the products, which are available without a prescription.

The warning letter the agency sent to Matrixx Initiatives, which sells Zicam, illuminates a corner of the health-care world we don’t often mention on the blog: homeopathy.

According to this explainer from the National Institutes of Health, homeopathy is based on “giving extremely small doses of substances that produce characteristic symptoms of illness in healthy people when given in larger doses.” (The active ingredient in Zicam is a form of zinc.)

The NIH says that individual studies of homeopathy have had varying results, but adds that “systematic reviews have not found homeopathy to be a definitively proven treatment for any medical condition.”

Zicam is marketed as a “homeopathic medicine.” As the FDA’s warning letter makes clear, “many homeopathic drug products are manufactured and distributed without FDA approval.” But, the letter adds, the agency can intervene when there’s evidence a homeopathic drug may pose a risk to consumers. That’s what it’s doing in this case.

The warning letter to Matrixx says that, because of the possible risks, the FDA will enforce its standard new-drug approval requirements for the intranasal Zicam products. That means Matrixx will have to demonstrate to the FDA’s satisfaction that “based on adequate and well-controlled clinical investigations … [the] product is safe and effective for each of its intended uses.”