The number of heroin users in the United States has reached the highest level in 20 years, a new international report found. Researchers call the heroin usage trend "alarming."

According to the World Drug Report 2016 from the United Nations (UN) Office on Drugs and Crime, there were approximately 1 million heroin users in the United States as of 2014. The latest numbers were nearly three times higher compared with the rates in 2003.

The research, which analyzes the health impacts of various substances such as cocaine, opiates, amphetamine and cannabis, found that the recent heroin-related deaths increased five times since 2000.

"There is really a huge epidemic [of] heroin in the U.S. It is the highest definitely in the last 20 years," said the report's chief researcher Angela Me who added that the trend is ongoing.

The latest report highlighted heroin as the world's deadliest drug. Moreover, its spiking number of users in the United States is becoming a particular concern.

"[The World Drug Report 2016] reveals heroin hike in some regions; a drug that kills more people than any other," wrote Yury Fedotov, the UN Drugs and Crime Office executive director, on Twitter.

In Central and Western Europe, heroin use as well as other drug overdose-related deaths spiked significantly in over the last two years, the report found.

According to Scott Krakower, an addiction expert and Zucker Hillside Hospital's assistant unit chief of psychiatry, there are many theories as to the cause of heroin use spike in the United States.

Krakower said that the biggest theory is the crackdown on OxyContin, Vicodin as well as other prescription drugs, which were often overprescribed. As the prescriptions of these drugs reduced, "heroin use went up."

OxyContin as well as other opioid painkillers are legal prescription drugs. Heroin is an illegal drug, and the spike in users and demands has likely led to the increase in supply coming from outside of the United States.

In recent years, new laws in the United States made it more difficult to abuse opioid prescription drugs such as oxycodone, whose painkilling effects are similar to heroin.

Under such laws, the pills' textures were changed, making them more difficult to crush and inject into a person's bloodstream. According to Me, the change led to a "partial shift" from opioids misuse to heroin.

Heroin is a powerful opioid that can lead to "pretty quick highs," said Krakower. This illegal drug can suppress breathing and can eventually lead to death.

The World Drug Report 2016 report is available online [PDF].

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