People in several states have fallen ill after receiving injections outside of doctors’ offices.

The Centers for Disease Control and Prevention said this week that it is coordinating an investigation into illnesses linked with injections of botulinum toxin, commonly called Botox, across several states.

The injections were administered in “nonmedical settings,” the C.D.C. said in a statement, which could include homes or cosmetic spas. The agency said that the source of the injections is currently “unknown and unverified.” The shots appeared to be counterfeit in at least some cases.

The Tennessee Department of Health said that four people in the state had gotten sick with botulism-like symptoms, including two who were hospitalized after receiving possible counterfeit injections. And health officials in Illinois have also warned health care providers to watch out for patients with symptoms of botulism after two people developed blurred or double vision, facial drooping, fatigue, hoarse voices and had difficulty breathing after injections of Botox or a possible counterfeit product.

Botulism is a rare but serious illness that occurs when a toxin attacks nerves in the body. Doctors treat the condition with antitoxin medications to prevent further damage.

Botox injections contain small doses of these toxins, which can relax muscles in the face to temporarily smooth wrinkles and fine lines. Approved injections use purified and sterile versions of the compound that meet Food and Drug Administration standards. “It’s in the right dose, it’s been well-studied,” said Dr. Mark Hamilton, a facial plastic surgeon in Indianapolis. “If you have some counterfeit product, all that’s thrown out the window.”

Counterfeit injections have been “a long-term problem,” said Shabbir Imber Safdar, executive director of the Partnership for Safe Medicines, a nonprofit coalition that advocates for protecting consumers against counterfeit and unsafe medicines.

“We’ve had some real harm,” he said. In 2005, for example, a husband-and-wife team of doctors pleaded guilty to distributing knockoff Botox. More than 1,000 patients had been injected with the substance.

Both patients in Illinois had received injections from a licensed nurse administering the shots “outside of her authority,” state health officials said. The C.D.C. said in a statement that “cosmetic injections should be an F.D.A.-approved product, administered by licensed providers and in licensed settings.”

Dr. Scot Glasberg, president of the Plastic Surgery Foundation, urged people seeking out Botox to see licensed professionals in medical settings.

“Do your homework,” he said. “Although Botox seems like a very easy procedure, clearly, as demonstrated, there are risks to it. That’s why you want to know who you’re going to.”