Some evidence is emerging that fentanyl, a powerful and highly addictive synthetic opiate, is replacing heroin and other drugs, particularly on the East Coast. The soaring production of heroin in recent years may also have accounted for the recent drop.

The area under poppy cultivation in Mexico reached a record high in 2017, rising 38 percent from the previous year, according to statistics from the Drug Enforcement Administration. Over the same period, heroin production increased by a similar percentage, rising to more than 122 tons in 2017 from about 89 tons the year before, perhaps creating a glut in the market, officials and experts say.

In the largely indigenous farming communities of the La Montaña region, however, there is only bewilderment about the fact that something so lucrative has suddenly become so worthless.

The numbers are stark: At the market’s peak in 2017, the farmers were selling their opium resin for as much as $590 per pound. It currently fetches no more than $50 per pound.

Mountain slopes that were once blanketed with the plant, its pink, purple and red flowers lending a burst of vibrancy to an otherwise dusty landscape, now lie fallow or have been replaced by subsistence crops — mainly corn.

There is widespread bewilderment about the cause of the precipitous drop in demand. The word “fentanyl” is met with puzzled looks. Few residents even admit to knowing that the opium resin is converted into drugs for consumption in the United States.

“We heard on the news that it was used in pills,” said Delfino Morán Ramírez, 43, a community leader in Ahuixotitla, another agricultural hamlet in the La Montaña region. “But beyond that, no.”