They call it the poor man’s methadone.

The epidemic of opioid addiction sweeping the country has led to another form of drug abuse that few experts saw coming: Addicts who cannot lay hands on painkillers are instead turning to Imodium and other anti-diarrhea medications.

The active ingredient, loperamide, offers a cheap high if it is consumed in extraordinary amounts. But in addition to being uncomfortably constipating, it can also be toxic, even deadly, to the heart.

A report published online in the Annals of Emergency Medicine recently described two deaths in New York after loperamide abuse. And overdoses have been linked to deaths or life-threatening irregular heartbeats in at least a dozen other cases in five states in the last 18 months.

Most physicians just recently realized that loperamide could be abused, and few look for it. There is little if any national data on the problem, but many toxicologists and emergency department doctors suspect that it is more widespread than scattered reports suggest.

As efforts to limit prescription opioids intensify, a handful of experts are concerned that more addicts might turn to loperamide — much as an alcoholic might resort to mouthwash when the Jim Beam runs dry.

“We’ve seen patients who have been on loperamide for months at a time,” said William Eggleston, the lead author of the recent report and a clinical toxicologist at SUNY Upstate Medical Center.

He added, “A subset of patients take it to get high, and other patients use it as a bridge” — meaning that if they cannot obtain heroin or morphine, they take loperamide to ease withdrawal symptoms like muscle pains, vomiting, diarrhea and nausea.

Sarah Peddicord, a spokeswoman for the Food and Drug Administration, said, “The FDA is aware of recent reports of adverse events related to the intentional misuse and/or abuse of the anti-diarrhea product loperamide to treat symptoms of opioid withdrawal or produce euphoric effects.”

After a review, she said, the agency “will take appropriate steps as soon as possible.”

The recommended dose of loperamide is safe. The standard daily dose of Imodium A-D is no more than four caplets, or eight milligrams. But lobe abusers — as they sometimes call themselves — have reported ingesting 100 2-milligram tablets daily for weeks.

In a case reported by Eggleston and his colleagues, a 24-year-old man experiencing opiate withdrawal took so much loperamide that he died. Toxicology analysis found more than 25 times the regular dose in his blood.

In another case, a 39-year-old man collapsed at home and was pronounced dead at a hospital. His family said he had once managed his opioid addiction with prescription buprenorphine, but had taken to medicating with anti-diarrhea drugs.

Anti-diarrhea medications are cheap, legal and can be bought easily in large quantities without raising suspicion. Costco sells 400 loperamide caplets for just $7.59.

Yet loperamide used to be a prescription drug and a controlled substance, in the same class as cocaine or methadone. The FDA approved it in 1976, and it became an over-the-counter drug in 1988.

Typically, loperamide acts on opioid receptors in the gastrointestinal tract and does not enter the central nervous system. At recommended doses, there is no high, and low potential for abuse. But large doses can produce a high, doctors say.

Some toxicologists argue that the sales of loperamide should be limited, much as the nonprescription drug pseudoephedrine was restricted a decade ago to help prevent the manufacturing of crystal meth.

“It’s time for someone to step in and regulate the purchasing of massive quantities,” said Dr. Chuck O’Connell, an emergency medicine physician and toxicologist at the University of California, San Diego, who said he had seen two loperamide overdoses.

“The average person doesn’t need 400 tablets of loperamide weekly,” he said. “I’ve used a handful in my whole life.”

In the journal HeartRhythm Case Reports, he described a 28-year-old woman who said she had taken 400 to 600 milligrams of loperamide daily for months. An electrocardiogram showed dangerously irregular heartbeats and abnormal electrical conduction through her heart.

After repeated blackouts, she sought medical attention at a hospital. While there, unknown to doctors, she was still taking 100 tablets of loperamide a day from a private stockpile.

After she transferred to UCSD and confessed her habit, O’Connell asked her to stop. In a few days, an electrocardiogram showed her heart normalizing, and the fainting subsided.

“If you take enough, it rushes the gate and some penetrates the blood-brain barrier,” O’Connell said. “Once it crosses the barrier, it can act on the central nervous system and you get euphoric effects.”

Some users complain the high does not compare to that produced by opioids. “You can definitely get high from it, and even kill yourself with it,” a commenter wrote in 2013 on Bluelight, a website where people discuss drug use.

The high was “not worth the health risks, whatever they are,” the commenter wrote.

Another commenter cataloged loperamide’s downsides — the need to continually take stool softeners, for one — but wrote that the medication took away the misery of opioid withdrawal: “Don’t wish to be dead … so that’s a plus.”

Loperamide abuse may go undetected in emergency departments, experts warn, because routine drug screens cannot detect it.

“The urine toxicology we do in our hospital doesn’t look for loperamide, so it’s possible we missed cases,” said Amitava Dasgupta, a toxicologist at the University of Texas Health Science Center at Houston.

Some loperamide abusers arrive at hospitals lethargic or not breathing, as if they had overdosed on heroin. Naloxone, an anti-opioid drug, may be given.

“When a drug screen comes back negative, emergency room staff may assume the test was faulty, or by that time, if the patient is responsive, they may write it off as nothing,” said Dr. Jennifer Dierksen, a pathologist at the University of Texas Health Science Center at Houston.

In one case at the hospital, opioid abuse was suspected after a 19-year-old Texan was found dead at home with a distended bladder full of urine. But a drug screen was clean. So Dasgupta used a test known as liquid chromatography-mass spectrometry to pinpoint loperamide as the culprit.

All cases of cardiac problems associated with the misuse or abuse of loperamide should be reported to the FDA’s Medwatch online registry. But not all physicians do so.

“The more people sounding the alarm, the more likely the FDA will take the problem seriously and take action,” Eggleston said. “A first step would be legislation or regulation that places the items behind the counter.”

Johnson & Johnson, Imodium’s manufacturer, did not respond to requests for comment.