Security forces in both countries have clamped down in recent months on exports of the psychostimulant, produced in swathes of Syrian and Lebanese territory where government oversight is lax or non-existent.
"When the Syrian crisis started, Lebanon and Syria were transformed into a gateway to smuggle captagon," a Lebanese security source told AFP on condition of anonymity.
"The substance wasn't invented in the past five years -- but that's when smuggling operations flourished, so Lebanon became an exporting country," he said.
Captagon is classified by the UN Office on Drugs and Crime as an "amphetamine-type stimulant" and usually blends amphetamines, caffeine and other substances.
On December 30, Lebanese authorities said they, in coordination with Saudi Arabia, had seized 12 million captagon capsules and arrested the "mastermind" of a cell exporting them to the Gulf.
And in October, Lebanon arrested a Saudi prince and four other Saudi nationals for attempting to smuggle out nearly two tonnes of captagon via Beirut airport, in one of the country's largest busts.
General Maamun Ammuri, head of Syria's Drug Enforcement Agency, said authorities had seized shipments amounting to 24 million captagon capsules in 2015 alone.
Some five million capsules were discovered en route from Syria's Mediterranean port of Tartus to Kuwait, and other "large amounts" were confiscated along the Lebanese and Turkish borders, Ammuri said.
In war-torn Syria, authorities say captagon production occurs largely in areas outside government control, including northern Aleppo and on the outskirts of the capital.
"Our documents indicate that drug traffickers are terrorist groups with two goals: first, to spread this poison in areas under the control of the Islamic State group to fund themselves and buy weapons," said Ammuri.
"And second, for its fighters to use so that they become numb to their own criminal activities, like beheadings and executions," he added.
A former opposition fighter who is now in Lebanon as a refugee said he used to take captagon with his fellow fighters "to eliminate fatigue and fear so we could stay up longer."
Signup to our newsletter and follow us on Facebook and Twitter!
"It made us brave and gave us extraordinary energy," he told AFP.
Another Syrian rebel said that "IS and (Al-Qaeda affiliate) Al-Nusra Front, as well as Islamist groups, forbid the use of these drugs because they're against Islamic law."
A third admitted that opposition factions produce captagon, but said it was strictly to generate funds for rebel groups and members were banned from partaking.
$5-10 a pop
In Lebanon, factories churning out captagon are largely based along the restive border with Syria, including the towns of Arsal, Flita, and Brital in the east and Wadi Khaled in the north, the security source said.
"Captagon factories don't need a lot of space. You could produce millions of capsules in a minivan without making any noise," a second security source in Lebanon said.
A captagon producer in Lebanon's vast east Bekaa Valley, on condition of anonymity, said dealers typically buy captagon from him in packages of 200 capsules.
"Producing captagon requires amphetamine, which you can get from ketone, in addition to rubbing alcohol and citric acid," he said.
He dries the ingredients and transfers them to a pressing machine typically used to make hard candy.
From there, the drugs are transported to Lebanon's seaside airport to be smuggled to eager consumers, mostly in the Gulf.
"Captagon isn't popular in Lebanon, and the level of demand does not compare to the other kinds of drugs because of its price -- between $5 and $10 per pill," the Lebanese security source said.
"Gulf countries are the number one captagon consumers, especially Saudi Arabia. Most of the smuggling operations that are busted are heading to Saudi.
"The reason they consume this drug in particular is their belief that it is a sexual stimulant," he added.
The Saudi interior ministry announced in November that it had seized more than 22.4 million capsules of amphetamines.
Convicted drug traffickers in Saudi Arabia are sentenced to death.