CHICAGO — Consumers shopping for medications on the Internet often are getting convenience, a good price and the cloak of privacy, but they may not be getting the real thing.

A burgeoning multibillion-dollar industry of counterfeit drugs — ranging from AIDS and cancer medications to antidepressants and sexual enhancers — is keeping regulators busy and leaving the public vulnerable.

These medicines can deliver too little, too much or none of the active ingredient — or the wrong one — and sometimes are adulterated with dangerous chemicals or contaminated by unsanitary manufacturing or storage conditions.

The FDA in recent years has confiscated millions of dollars’ worth of counterfeit medicine. The agency, in partnership with international regulatory, customs and law enforcement agencies from 100 countries, shut down thousands of Internet pharmacies selling illegal drugs and seized about $10.5 million worth of pharmaceuticals during a weeklong crackdown on counterfeit and unapproved medications that launched in late September.

But such enforcement action isn’t nearly enough to stop the proliferation of phony medicine, FDA officials concede.

“This is a drop in the bucket,” said Ilisa Bernstein, director of the Office of Compliance in the FDA’s Center for Drug Evaluation and Research. “We don’t know how many websites are out there, but there are a lot more. We may have some impact on these 4,100 websites, but they can pop up days or weeks later using another URL and another way to deceive consumers.”

It is illegal in the United States to sell medication without a valid prescription. All U.S. pharmacies, including those offering drugs online, must be licensed in the state where they are based or where they do business.

Foreign pharmacies can be licensed in the United States only if they follow all state and federal laws and they distribute only FDA-approved products.