A synthetic drug that's more potent than heroin is turning up in the U.S. — and is being legally* sold — because Chinese labs are effectively exploiting a loophole in export bans and drug laws.

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Fentanyl, a synthetic opiate that's prescribed for chronic pain, is classified as a Schedule 2 drug in the U.S., meaning that it's technically illegal to sell in the United States. However, by slightly modifying the drug's chemical structure — changing just one molecule of the drug — Chinese labs are able to circumvent the China's export ban on synthetic drugs.

Now a new version of the drug has made its way to the U.S.

The new version of the pill, which is called furanyl fentanyl, isn't explicitly prohibited under the Controlled Substances Act, the federal list of banned drugs. As an "analog" of the banned drug, however, it is still illegal to sell under the Analog Act, a DEA spokesperson told ATTN:. That said, the drug can be legally imported from China for the time being.

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The federal agency said that it plans to move quickly to formalize the U.S. ban on the new fentanyl product, but the effectiveness of that strategy has been repeatedly challenged by synthetic drug distributors in the past.`

"We saw that happen three of four years ago," DEA spokesperson Melvin Patterson said.

Back then, synthetic drugs such as K-2 and Spice (which are designed to mimic the effects of marijuana) were being rapidly exported from Chinese labs and sold across the United States. The DEA struggled to keep up with the dozens of derivatives that began to appear in headshops, and President Obama responded in 2012 by signing the Synthetic Drug Abuse Prevention Act, which added synthetic marijuana and stimulants to the banned substances list.

Fentanyl abuse is a growing problem in the U.S.

"Furanyl fentanyl was recently identified as the cause of death in the fatal overdose of a 30-year-old man in Naperville, Ill., a suburb west of Chicago," STAT, a media company that focuses on health and medicine, reports. "It appears to be the first public reporting of a case in which the new version of fentanyl caused a deadly overdose."

While the new version of fentanyl has only been blamed for one death in the U.S., the standard version of the synthetic drug (acetyl fentanyl) is part of a growing public health crisis, the opioid epidemic. Approximately 18,000 people died from opioids in 2014, and though it is uncertain how many fatal fentanyl overdoses occurred during that timeline, "its presence seems to be rising sharply, which means that, given its potency, deaths from the drug are also rising," Forbes reports.

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*Furanyl fentanyl is not explicitly prohibited under the Controlled Substances Act, but acetyl fentanyl is. The DEA says it is able to prosecute people who sell the new version of acetyl fentanyl under the Analog Act.