According to the U.N.'s World Drug Report 2016, the number of heroin users in the United States reached around one million in 2014, almost three times as many as in 2003. Heroin-related deaths there have increased five-fold since 2000.
"There is really a huge epidemic (of) heroin in the U.S.," said Angela Me, the chief researcher for the report which was released on Thursday.
"It is the highest definitely in the last 20 years," Me said, adding that the trend was continuing.
The rise could be linked to U.S. legislation introduced in recent years which makes it harder to abuse prescription opioids such as oxicodone, a powerful painkiller that can have similar effects to heroin, Me said.
Another reason for the increase in the use of heroin, which in the United States mainly comes from Mexico and Colombia, is greater supply that has depressed prices in recent years, Me said.
The United States has also seen a spike in deaths related to fentanyl, a synthetic opioid 50 times stronger than heroin and 100 times more so than morphine, according to the U.S. Centers for Disease Control and Prevention.