NARATHIWAT, THAILAND -- The drug of choice in the violence-plagued provinces of southern Thailand is a red-veined leaf from a tree that grows abundantly in the steamy tropical jungles here but has been declared illegal by the Thai government.

Chewing the leaves of the tree, kratom, which is in the same family as the coffee tree, was until recently a fading tradition among farmers and rubber tappers who sought an energy boost and stamina under the oppressive sun.

Now the spreading popularity of a narcotic cocktail based on kratom is creating a sharp increase in demand for the leaf. Young people sneak into protected forests and smuggle out duffel bags stuffed with the feather-shaped leaves. The authorities are attempting to counter the trafficking by staging sting operations, but they complain that the fines for offenders are too lenient -- and that the young people are too nimble to pursue through the jungle.

''It's an epidemic,'' said Srisompob Jitpiromsri, the associate dean at Prince of Songkla University in the southern city of Pattani. ''Kratom use has spread all over the place.''

Mr. Srisompob has conducted surveys on behalf of Thailand's Office of the Narcotics Control Board that track the use of kratom in the three troubled provinces along the border with Malaysia. His most recent survey, carried out this year among 1,000 teenagers, found that 94 percent of respondents used kratom.

In other studies, Mr. Srisompob found that drug use topped the list of ''most serious problems'' cited by villagers -- ranking before the region's shadowy Muslim insurgency, which has killed more than 5,000 since 2004. Kratom is the most popular drug in the deep south, beating methamphetamines and heroin.

''Drug use and poverty are always at the top of the list of most serious problems,'' Mr. Srisompob said. ''The insurgency is third.''

Consuming kratom has been illegal in Thailand since 1943, but until recently the law was only loosely enforced. Chewing the leaves has only a mild, barely perceptible stimulating effect. The cocktail, which is typically made by boiling kratom leaves, straining out the liquid and mixing it with cough syrup, Coca-Cola and ice, sends users into a sleepy torpor.

''It's not tasty if it's not cold,'' said a 26-year-old daily user of kratom who wanted to be identified only by his nickname, Mung.

Mung said he enjoyed the calming effect of the drug. In the predominantly Muslim province in which he lives, kratom carried less stigma than alcohol, said Mung, who is Muslim himself. Ninety-eight percent of the users in Mr. Srisompob's survey identified themselves as Muslim.

''Older people aren't angry if you boil kratom leaves because it's considered medicine,'' Mung said. Unlike other drugs, there are few visible signs that someone has consumed kratom -- no dilated pupils or alcohol breath.

Another reason for the drug's popularity is price. ''It's cheaper than other narcotics,'' said Maj. Gen. Choti Chavalviwat, the police commander in Narathiwat Province. Twenty leaves, enough to concoct a kratom cocktail for several people, cost the equivalent of $3.

The cocktail, which is widely known as 4x100 for reasons that are unclear, is rarely consumed outside the three provinces, General Choti said. Last year the police seized 415 liters, or 110 gallons, of liquid kratom and 1.3 tons of kratom leaves in the three provinces.

Some Thai officials draw links between drug trafficking, including kratom, and the insurgency.

Lt. Gen. Udomchai Thammasarorat, the commander of forces in southern Thailand, said during a news briefing last year that most violence was linked to drugs and smuggling across the border with Malaysia. ''I would say only 20 percent of the incidents here are caused by separatists, and around 80 percent are other crimes,'' he said.

The Thai-Malaysian border sits along a major trafficking route for methamphetamines and heroin that originate in Myanmar. And there is a widespread black market centered on smuggling diesel fuel from Malaysia, where it costs less because of government subsidies.

Yet the links between drugs and the insurgency are disputed by many experts and law enforcement officials.

General Choti said that if there were a link between drugs and the insurgency, it was very weak. ''Religion, history and ethnicity drive the insurgency,'' he said. The majority of the 1.9 million inhabitants of the three provinces are Malay Muslims who speak a dialect of Malay used across the border in the Malaysian state of Kelantan.

The ultimate goals of the insurgency are unclear. And unlike many terrorist acts elsewhere in the world, the nearly daily attacks in the three provinces, many targeting symbols of the Thai state, occur without groups or individuals taking responsibility.

Several years ago, Thai anti-narcotic officials sought to quantify the link between drugs and the insurgency, according to Mr. Srisompob. They compared a list of about 9,000 people who had gone through drug rehabilitation programs with 8,000 people suspected of being involved in the insurgency.

''They came up with about 2 or 3 percent on the lists who overlapped,'' Mr. Srisompob said.

Insurgents rarely recruit heavy drug users to their cause, Mr. Srisompob said, because their addiction makes them unreliable foot soldiers in the war against the Thai state.

But Mr. Srisompob sees other, more subtle links between the insurgency, drugs, poverty and unemployment.

''It allows the leaders of the insurgency to say, 'You see what Thai society is doing to us. They are trying to undermine Muslim society,''' he said.

Kratom is addictive, according to Savitree Assanangkornchai, the head of the epidemiology unit at Prince of Songkla University in the southern Thai city of Hat Yai. Users must increase the amount of drugs they take to obtain the same effect, and attempting to quit will result in withdrawal symptoms, she said.

But experts say more research is needed on kratom's potential medicinal use. The tree is helpful in treating diabetes, according to local lore.

The Office of the Narcotics Control Board has proposed that kratom be legalized, according to a report on kratom published by the Transnational Institute, which is based in the Netherlands.

For now, the Thai policy has been to eradicate the tree wherever it is found. In recent months, the Thai police have stepped up their campaign of cutting down trees across the country. But this policy has created tensions between law enforcement and those charged with protecting the environment.

The largest collection of kratom trees can be found in a protected forest in neighboring Satun Province, a spot accessible on foot by following a path that travels along a river and then passes through a large cave. Hundreds of kratom trees thrive in a scenic valley surrounded by limestone cliffs.

The authorities have ordered the trees felled, but the guardians of the forest are balking.

Narong Kaewsen, a park ranger at the Satun reserve, said the kratom trees were so resilient that cutting them down would not suffice. All around him, kratom seedlings were pushing through the muddy soil. Destroying the trees, which are spread over about 30 acres, would require large amounts of herbicide, he said. ''At the very least it will harm the water, animals and plants,'' Mr. Narong said. ''They will also die.''

For now, Mr. Narong and other local officials are trying to stop kratom trafficking by intercepting the young people who prowl the forests, often at night, in search of the leaves.

Panya Tonoon, the headman of the local village, intercepts young traffickers several times a month. ''They often just run into the jungles,'' Mr. Panya said. ''Sometimes we chase them.''

Poypiti Amatatham contributed reporting.