The US Food and Drug Administration has stopped a California company from continuing to inject stem cells mixed with the smallpox vaccine into the tumors of cancer patients.

Last Friday, US marshals seized five vials of the smallpox vaccine from San Diego-based StemImmune Inc, which was using them as part of an unproven method for treating tumors. (The FDA found out about the vials from previous StemImmune inspections.)

Each of the vials included 100 doses of the vaccine, and one of them was partially used. The vaccine is not commercially available and is only reserved for people who are at very high risk for developing smallpox, so it’s not clear how StemImmune got the vials to begin with. The company was mixing it with stem cells that come from body fat and injecting into the tumors of patients at California Stem Cell Treatment Centers in Beverly Hills and Rancho Mirage.

The problem wasn’t with the vaccine itself, which has been used in cancer trials before. And, luckily, the vaccine is not made from the actual smallpox virus and cannot give anyone the disease. The problem is that mixing them with stem cells is not approved and could be dangerous. Cancer patients often have weaker immune systems, and use of this vaccine could have caused problems such as serious swelling of the heart. In addition, if someone is not vaccinated and comes in contact with one of these patients, they, too, are at risk for developing this dangerous swelling. For now, it’s unclear whether people have been harmed by this procedure.

Smallpox is a highly contagious disease that dates back to ancient Egypt and once killed three of every 10 people who contracted it. The World Health Organization started a global campaign to eradicate smallpox in 1959, and the last case was in Bangladesh in 1975. The smallpox vaccine itself — first developed in 1796 — was the first successful vaccine.

StemImmune said in a statement that it “is fully cooperating with the FDA about the development of its stem cell-based investigational cancer therapy.”

The FDA is encouraging consumers who have tried the treatment and had bad effects to use its MedWatch Adverse Event Reporting program.

Update: This post has been updated with StemImmune’s response.