The discovery that a fake version of the widely used cancer medicine Avastin is circulating in the United States is raising fears that the multibillion-dollar drug-counterfeiting trade is increasingly making inroads in this country.

The criminal practice has largely been relegated to poor countries with lax regulations. But with more medicines and drug ingredients for sale in the U.S. being manufactured overseas, American authorities are afraid more counterfeits will find their way into this country, putting patients’ lives at risk.

The Avastin discovery follows other U.S. counterfeiting cases, involving such drugs as the cholesterol medicine Lipitor and the weight-loss pill Alli.

“We do know there are counterfeits continuing to try and make their way onto the U.S. supply chain,” said Connie Jung, an associate director in the Food and Drug Administration’s office of drug security.

The FDA announced Tuesday it is investigating fake vials of Avastin that were sold to at least 19 doctors and clinics — 16 sites in California, two in Texas and one in Chicago. Tests showed the vials did not contain the active ingredient in Avastin.

The contents of the vials are still being analyzed. The FDA said it has not received any reports of patients who were harmed.

FDA officials said the counterfeit Avastin was imported from Britain and distributed by Volunteer Distribution, a wholesaler based in Gainesboro, Tenn. British regulators notified the FDA about the products in December, but the agency didn’t confirm they were fake until last week.

The FDA gave assurances Wednesday that the United States remains one of the most secure pharmaceutical markets in the world. But cancer doctors scrambled to check their records.

Because Avastin treatments are spaced one to two weeks apart, it is unlikely that someone would get more than one infusion from the same vial. And because these are people facing a life-threatening disease, it is hard to say whether missing one treatment with the real drug would compromise their care. Gauging harm from a counterfeit cancer treatment is nearly impossible, said Dr. Robert C. Young, a consultant and former president of the Fox Chase Cancer Center in Philadelphia.

Counterfeits have traditionally been more of a concern in developing regions like Asia and Latin America, where as many as 30 percent of drugs sold are fake, according to the World Health Organization.

The group estimates just 1 percent of drugs dispensed in the United States and other developed nations are fake. But incidents of counterfeiting reported by drugmakers have increased over the decade to more than 1,700 last year, though only 6 percent of those were in the United States.