But research by Reuters shows fentanyl is creeping further into Mexico now, although the scale of consumption may be clouded by a lack of data and testing.

That gap in knowledge concerns Maria Elena Ramos, director of the social service organization "Compañeros".

"We don't have evidence of fentanyl, so this worries us a lot, because if it passes through Mexico, some of it must end up here. And if it's left in the streets, this affects people who are in difficult conditions, homeless people, those without the means to go to an emergency services."

Mexican authorities have classified fentanyl as an "emerging drug" because of an uptick in users seeking treatment, with related figures remaining relatively low.

According to most recent available data, opioid users made up less than 2% of the some-168,000 people who sought drug treatment in 2022.

And in 2021, fewer than two dozen opioid-related deaths were officially logged.

Compare that to the U.S., where potent synthetic opioids like fentanyl cause tens of thousands of deadly overdoses per year.

But Reuters has found that more than a third of Mexico's 32 states lack equipment to detect whether the drug is present in corpses.

Mexican health officials have publicly acknowledged gaps in the data.

The nation's addiction commission director, Evalinda Barron, who also oversees mental health, says it comes down to a matter of resource priority.

She admits fentanyl is a concern, but insists Mexico is less predisposed to an epidemic, because it does not have the same history as the U.S. of prescription pain medication abuse and heroin consumption.

"The use of fentanyl is not a public health problem now. We hope it won't be. We must bear in mind the opioid crisis started in other countries and it had to do with the use of drugs for pain. In Mexico, fentanyl consumption takes place at border zones because of that phenomenon."

However, some researchers argue that what data they can collect point to a possible growing epidemic.

Fentanyl showed up in nearly a quarter of corpses tested since June 2022 in the border town of Mexicali, according to state data.

There are concerns of people consuming it unintentionally - with studies finding fentanyl cut into heroin, as well as party drugs like cocaine and MDMA.

And as analysts say criminal groups are diversifying their routes, studies show the opioid emerging further away from border areas, all the way in central regions like Mexico City.

Julian Rojas, who works with Compañeros, says the authorities' dismissive attitude is a problem, especially since it means people don't have access to life-saving drugs in cases of overdoses.

"Talk that in Mexico there isn't fentanyl consumption or there's minimum level consumption - this puts the user at a greater risk as it continues to make them invisible. The government doesn't recognize the use of Naloxone as a drug to revert opioid overdoses. This puts users of drugs that circulate on our country's streets in a more vulnerable condition."

Still, officials are sounding the alarm, including through a public information campaign warning of the powerful drug's risks.

Mexico's president said in January that while fentanyl consumption was low, the country "has to be careful of it", adding that he was seeking more information about its use in different states.