It's official: stem cells are drugs. At least, that's the opinion of the US district court in Washington DC, which has ruled that the Food and Drug Administration (FDA) has the authority to regulate clinics offering controversial stem cell therapies.

Treatments in which stem cells are harvested from bone marrow and injected straight back into the same patient are deemed part of routine medical practice - not regulated by the US government. But if the cells are subjected to more than "minimal manipulation", the FDA maintains that the therapy becomes a "drug", which must be specifically approved for use.

It was on this basis that in 2008 the FDA began moves to shut down Regenerative Sciences, a clinic in Broomfield, Colorado, that treats orthopaedic problems using a stem cell therapy called Regenexx.

Regenerative Sciences challenged the FDA's authority to regulate its activities, setting the stage for a legal fight. In 2010, the FDA sought an injunction to take Regenexx off the market. This has now been granted in the court's ruling.

Christopher Centeno, medical director of Regenerative Sciences, vows to appeal. "This is really round one," he says. "Our position remains that a patient's cells are not drugs."

Regenexx consists of mesenchymal stem cells, which give rise to tissues including bone and cartilage, taken from a patient's bone marrow and grown in culture for about two weeks. Centeno has published a series of case reports describing its use to treat joint problems - but no controlled clinical trials.

"I think it's a good ruling, and I'm glad to see that that the FDA has exercised its muscle on the case," says Christopher Scott, who heads the Program on Stem Cells in Society at Stanford University in California.

Scott hopes that the FDA will now step up its efforts to regulate other clinics offering unproven stem cell therapies. These include Celltex of Sugar Land, Texas, which rose to prominence after Texas governor Rick Perry was injected with stem cells supplied by the company to aid his recovery from back surgery.

Last month, the Houston Chronicle revealed that FDA inspectors had found multiple violations of good manufacturing practice at Celltex.