Nicknamed "US$5 insanity'' - due to the quick and cheap high it gives - the drug has been linked to a spate of high-profile cases in the US in recent weeks which have seen people on Flakka go on sprees of demented behaviour.

The drug, and the many synthetic chemical variants on the market, is completely legal in mainland China and is just a mouse click away through companies and websites.

"It's a big deal. It is the new wave of drugs," said a senior law enforcement source working on global narcotics issues.

"It will inundate the US for the next several years."

Designer drugs have proven a headache for governments worldwide, who have struggled to keep legislation in line with the rapid manufacture of the narcotics - often at a rate of more than one a day.

The growing number of these drugs was the focal point of a UN conference in Bangkok last month. China's delegation was led by An Guojun, deputy secretary general of the National Narcotics Control Commission.

Despite the threat posed by the likes of Flakka, a lack of legislation means the potentially lethal substances remain legal in mainland China.

Corrupt chemists there are now targeting the potentially lucrative - and illicit - US market, and have set up online sites to take orders from traffickers and users, according to police.

The narcotics are smuggled out illegally through the mail or sea cargo.

"Flakka is one of a multitude of chemicals produced in China with the intent to exploit drug loopholes at the expense of Americans," the law enforcement source said.

"If it's not illegal, China can't honour countries requests to do law enforcement. Flakka might as well be mascara."

The US has seen a spike in cases in recent weeks, including several incidents in Florida.

One user, claiming to be the mythical Norse god Thor, attempted to have sex with a tree. Another ran naked down a busy street, convinced he was being chased by rabid dogs, according to media and police reports.

Created from a synthesised version of Khat leaves, Flakka is said to induce extreme euphoria and a feeling of superhuman invincibility, although it has been linked with extreme aggression and psychosis.

It is closely related to so-called "bath salts," which surged in popularity in the US in 2011.

Tun Nay Soe, programme coordinator with the UN Office on Drugs and Crime in Bangkok, said legislation was needed to stop the spread of such narcotics.

"There is no legislation for these new psychoactive substances. But that is understandable as few have been thoroughly researched," he said.

More than 500 new forms of these drugs have been detected, he added.

"The rate of increase is not welcome news," he said.

Some jurisdictions, such as the US and Hong Kong, have already adopted broad enough regulations to prevent the spread of most of these substances.

However, in mainland China - where few citizens actually consume Flakka - moves to improve legislation have been bogged down in bureaucracy.

"Nobody believes China is purposefully dumping this stuff. It's just the bureaucratic requirements to make things illegal take time," said the source.

Nay Soe of the UN office echoed the sentiments, saying: "China is good at reacting to issues. Once they have been requested to cooperate, they will launch investigations and put in control measures.

"If Flakka is a problem, they will control it," he said.

Besides "rare occasions" when the substance transits the city, authorities do not believe Hong Kong has a key role in the Flakka trade.

"Like other international trading and transport hubs, the heavy traffic and passenger flow has posed challenges in combating smuggling activities. Yet Hong Kong maintains rigorous enforcement against drug trafficking," said a spokesman for the Customs department. He denied the city was drug transit centre.

Since January, the department cracked 67 cases, involving nearly 100kg of Flakka or similar substances, at the airport, the spokesman said.

However, the city has been named in one ongoing court case in Florida. Kevin Bully, 25, and Jaime Lewis, 22, stand accused of importing multiple packages of the controlled substance into the US from a "chemical company in Hong Kong, China," according to court documents.

The pair were charged last month after a sting operation caught them accepting packages filled with the illicit substance.

According to court documents, the Chinese chemical company was using Aramex, a Dubai-based logistics firm, to transport the chemicals to buyers in the United States.

Aramex did not reply to inquiries about its alleged role in the trade.

Legal highs that pose global threat

Known more informally as "designer drugs" or "legal highs", they are defined by the United Nations as substances of abuse, either in pure form or a preparation, that are not controlled under international drug covenants but may still pose a public health threat. Many mimic the effect off harder drugs that are under international control.

They are not under international control. However, some countries have established regulations that are broad enough to capture and control new variations as they are created. For example, the law in Hong Kong prohibits Flakka, despite it being legal and available on the mainland.

According to Tun Nay Soe, an expert on new psychoactive substances with the United Nations Office on Drugs and Crime, they can be constantly modified with various chemicals to keep them outside the law. Thorough research needs to be done before legislating and this can prove difficult as new variants are emerging every day, he says.

New psychoactive substances are now considered a global phenomenon. All regions have been affected by the drug, while 70 out of 80 countries surveyed recently by the UN have reported cases.

Drugs have been synthesised for many years. However, officials and law enforcement officers are now worried about the rapid increase in the pace the narcotics are being manufactured. Nay Soe says more than 500 unique new psychoactive substances have been detected.

Very easily online in countries such as China, where they are legal. A simple search for Flakka brings up a host of websites where it can be purchased. Many users in the US have been buying Flakka through companies in China and then smuggling it into North America.

Like most drugs, they can be adulterated with dangerous chemicals during the manufacturing process, creating potential dangers for users. Drugs such as Flakka and bath salts have also been linked with heart palpitations, hallucinations and aggressive behaviour.