As The Washington Post's Liz Sly recently noted, the war in Syria has become a tangled web of conflict dominated by "al-Qaida veterans, hardened Iraqi insurgents, Arab jihadist ideologues and Western volunteers."
On the surface, those competing actors are fueled by an overlapping mixture of ideologies and political agendas.
Just below it, experts suspect, they're powered by something else: Captagon.
A tiny, highly addictive pill produced in Syria and now widely available across the Middle East, its illegal sale funnels hundreds of millions of dollars back into the war-torn country's black-market economy each year, likely giving militias access to new arms, fighters and the ability to keep the conflict boiling, according to the Guardian.
"Syria is a tremendous problem in that it's a collapsed security sector, because of its porous borders, because of the presence of so many criminal elements and organized networks," the U.N. Office on Drugs and Crime (UNDOC) regional representative, Masood Karimipour, told Voice of America. "There's a great deal of trafficking being done of all sorts of illicit goods - guns, drugs, money, people. But what is being manufactured there and who is doing the manufacturing, that's not something we have visibility into from a distance."
A powerful amphetamine tablet based on the original synthetic drug known as "fenethylline," Captagon quickly produces a euphoric intensity in users, allowing Syria's fighters to stay up for days, killing with a numb, reckless abandon.
"You can't sleep or even close your eyes, forget about it," said a Lebanese user, one of three who appeared on camera without their names for a BBC Arabic documentary that aired in September. "And whatever you take to stop it, nothing can stop it."
"I felt like I own the world high," another user said. "Like I have power nobody has. A really nice feeling."
"There was no fear anymore after I took Captagon," a third man added.
Captagon has been around in the West since the 1960s, when it was given to people suffering from hyperactivity, narcolepsy and depression, according to a Reuters report published in 2014. By the 1980s, according to Reuters, the drug's addictive power led most countries to ban its use.
The United State classified fenethylline ("commonly known by the trademark name Captagon") as a Schedule I drug under the federal Controlled Substances Act in 1981, according to the National Criminal Justice Reference Service
Still, the drug didn't exactly disappear. VOA notes that while Westerners have speculated that the drug is being used by Islamic State fighters, the biggest consumer has for years been Saudi Arabia. In 2010, a third of the world's supply -- about seven tons -- ended up in Saudi Arabia, according to Reuters. VOA estimated that as many as 40,000 to 50,000 Saudis go through drug treatment each year.
"My theory is that Captagon still retains the veneer of medical respectability," Justin Thomas, an assistant professor of psychology and psychotherapy at the UAE's Zayed University and author of "Psychological Well-Being in the Gulf States," told VOA in 2010. "It may not be viewed as a drug or narcotic because it is not associated with smoking or injecting."
Five years later, production of Captagon has taken root in Syria - long a heavily trafficked thoroughfare for drugs journeying from Europe to the Gulf States -- and it has begun to blossom.
"The breakdown of state infrastructure, weakening of borders and proliferation of armed groups during the nearly three-year battle for control of Syria, has transformed the country from a stopover into a major production site," Reuters reported.
"Production in Lebanon's Bekaa valley -- a traditional centre for the drug -- fell 90% last year from 2011, with the decline largely attributed to production inside Syria," the Guardian noted.
Cheap and easy to produce using legal materials, the drug can be purchased for less than $20 a tablet and is popular among those Syrian fighters who don't follow strict interpretations of Islamic law, according to the Guardian.
Doctors report that the drug has dangerous side effects, including psychosis and brain damage, according to the BBC.
Ramzi Haddad, a Lebanese psychiatrist, told Reuters that the drug produces the typical effects of a stimulant.
"You're talkative, you don't sleep, you don't eat, you're energetic," he said.
But one secular ex-Syrian fighter who spoke to the BBC went further, saying the drug is tailor-made for the battlefield because of its ability to give soldiers superhuman energy and courage:
"So the brigade leader came and told us, 'this pill gives you energy, try it,' " he said. "So we took it the first time. We felt physically fit. And if there were 10 people in front of you, you could catch them and kill them. You're awake all the time. You don't have any problems, you don't even think about sleeping, you don't think to leave the checkpoint. It gives you great courage and power. If the leader told you to go break into a military barracks, I will break in with a brave heart and without any feeling of fear at all - you're not even tired."
Another ex-fighter told the BBC that his 350-person brigade took the pill without knowing if it was a drug or medicine for energy.
"Some people became addicted to it and it will damage the addicts," he said. "This is the problem."