(Associated Press) -- Imitation drugs have made their way into the regular-drug distribution chain that stocks legitimate pharmacies in two recent cases. While the vast majority of U.S. drugs are fine, there has been an increase in counterfeit drug cases. This potential threat to patients' lives, once a threat mostly in developing countries, is increasingly occurring in the U.S.

Undercover agents recently staked out a Chevron gas station in Florida, and witnessed a man hand $36,500 to his partner in a car. Instead of cocaine or heroin, the man was arrested for selling fake versions of the lifesaving medication Procrit. The Food and Drug Administration (FDA) intercepted the fake Procrit (1,800 vials filled with bacteria-tainted water) before it reached patients who depended on the blood-boosting injections. FDA believes that it intercepted all of the fake Procrit in this case.

Others have not been so lucky.

Another case still under investigation involves 130,000 bottles containing fake Lipitor, the top-selling cholesterol medicine. The imitation drug reached at least some patients.

"This is work of very good and very, very experienced counterfeiters," explains Miami U.S. attorney Marcos Jimenez of the Procrit case.

It is difficult to determine how often counterfeit drugs sell in the U.S., the country with the strictest pharmaceutical regulations. Since 1996, the FDA has investigated 71 counterfeit drug cases and arrested 43 involved people.

Legitimate Procrit sells for $1,700 wholesale per box of four vials. The counterfeit boxes were sold for $350 to *800 per box, yet cost no more than $1.50 to make. The fake medications looked real, including imitation holographic safety seals and numbers and stamps almost identical to those on the real medication. In this case, the man used a mix of store-bought and homemade machines to print labels just like the ones he was trying to imitate.

Such incidences have drug makers creating more secure packaging. In fact, Florida is about to tighten drug distribution requirements to make it harder for fake drugs to sneak into the chain, explains Lewis Kontnik of Reconnaissance International, a consulting firm working with FDA and industry on anti-counterfeiting measures.

Counterfeit drugs have long been problems in certain areas of the world. A study found that one third of malaria pills sampled in parts of Asia contained no trace of real medication. In fact, most U.S. counterfeiting has been of bulk ingredients shipped from another country. Yet a couple of years ago, FDA saw a trend toward more counterfeit finished brands like Procrit, Lipitor, Viagra, and the AIDS-related medications Serostism and Combivir.