In a shared duplex in Winooski, VT, a young man receives a half-dozen or so visitors within a few hours, one at a time, for a few minutes at a time.

By the end of the day, he's made his money back on a purchase of what's known colloquially as "bute," phenylbutazone, a pain killer for horses commonly abused by humans, once but no longer approved by the FDA for treatment of rheumatoid arthritis and gout.

Bute and other opioids — such as buprenorphine, oxycodone and codeine, which are available by prescription — are commonly abused by Americans who collectively consume 80 percent of the world's painkillers, ingesting, snorting and injecting more than 110 tons of addictive opiates every year, according to the American Society of Interventional Pain Physicians.

While opiate abusers tend to skew young, poor, and male, the rate of abuse by women is increasing markedly, and with it the number of American babies exposed to opiates in the womb.

"In the past decade, there has been a significant increase in opioid use during pregnancy, estimated to affect 5.6 per 1,000 births," Dr. Jonathan M. Davis, of Tufts Medical Center, reported Tuesday at meeting in New York City held by the Journal of the American Medical Association.

The number of babies born in the U.S. with "neonatal abstinence syndrome," a symptomology including dysfunction of the nervous system, gastrointestinal tract, and respiratory system, has tripled during the past decade. With mothers receiving buprenorphine or methadone as treatment for heroin addiction, 60 to 80 percent of newborns experience symptoms of the syndrome.

Davis said he and his colleagues have for the first time identified genetic predispositions for the development of the syndrome among babies, though they'd long known clinical factors such as maternal smoking, psychiatric medications, and breast feeding can affect the chances and severity of the illness.

Researchers studied 86 mother-infant pairs between mid-2011 and mid-2012 in Maine and Massachusetts, looking for correlations with three genes associated with risk for opioid addiction in adults, OPRM1, ABCB1, and COMT. The investigators genotyped DNA samples for single-nucleotide polymorphisms, correlating outcomes with each genotype.

Infants with a specific variation of the OPRM1 gene fared better than those with the AA genotype, requiring shorter hospitalizations than infants with the AA genotype, and were also less likely to receive treatment, at 48 percent to 72 percent. Likewise, infants with a variation of the COMT gene required shorter hospitalizations and fewer medications, while associations with the ABCB1 were insignificant.

"We really do think these genetic factors may be important," Davis said. "Sometimes, just one amino acid changing... can change the function of that protein."

Davis said advances in sequencing personal genomes would greatly benefit the treatment of infants born to mothers addicted to opioids, with such personalized genome tests costing only a few hundred dollars or so in the near future.

However, one of the problems with present treatments for the syndrome is a lack of FDA-approved drugs on the market, given high liability and low profit for such drugs — meaning some 90 percent of drugs used to treat the syndrome are used off-label.

"We often give infants narcotics [to treat the syndrome] but despite giving the maximum dosage, it's insufficient so we give other drugs," he said.

Davis said researchers interested in developing treatments for the syndrome should first attempt to replicate the findings of this study, which appears May 1 in the journal.