In 2009, Jackie Kim received a gastric bypass that shrunk her stomach and rerouted part of her small intestine. Within a year, she had lost 180 pounds and felt great.

Then her troubles with alcohol set in. It started with ordering wine in place of dessert at dinner. “At first I thought, ‘This is great, I don’t have to sit at the table twiddling my fork while everyone else is eating their crème brûlée,’” recalled Kim, a 44-year-old medical consultant living in St. Louis, Missouri.

But soon the occasional glass of wine turned into much more.

Kim spent the next two years fighting addiction. “Lots of scary stuff happened during those years,” she said. She regularly drank alone at home, two bottles of wine at a time. Before long, she was hiding bottles from her husband, driving while drunk, blacking out and discovering injuries she didn’t remember getting.

Her story is not uncommon. In 2012, a large study published in the Journal of the American Medical Association reported that the percentage of patients abusing alcohol increased from 7.6 percent before surgery to 9.6 percent two years after surgery — that’s potentially an additional 2,000 alcoholics each year in the United States. Since then, a growing body of evidence has corroborated these findings. The longest-running study suggests the effect persists even a decade after surgery.

Still, many patients today are unaware of the risk of alcoholism when they get a gastric bypass, and scientists themselves are not completely sure why the risk exists. One early theory was addiction transfer, which suggested that people might adopt new addictions after weight-loss surgery because they can no longer fulfill their food addictions. But more recent evidence suggests there may be an anatomical explanation: specific metabolic and hormonal changes triggered by gastric bypass that leave patients especially vulnerable to alcoholism but not other addictions. It’s also possible that both explanations are right — or neither.

“Whether it’s addiction transfer or something else going on, we really don’t know at this point,” said James Mitchell, a doctor and professor of neuroscience at the University of North Dakota. What’s certain, he said, is that the high rates of alcoholism in patients who have had a gastric bypass operation cannot be attributed to chance.

Doctors have long touted gastric bypass surgery as the gold standard for weight-loss operations. Of the 200,000 bariatric procedures performed in the United States each year, roughly 80 percent are gastric bypass surgeries. Research shows the surgery not only causes weight loss — 90 percent of gastric bypass patients keep off 50 percent of their extra weight even a decade after surgery — but also resolves related illnesses such as type 2 diabetes, heart disease and cancer, leading to a 40 percent overall reduction in mortality for gastric bypass patients.

But alcoholism could be a dark consequence of the surgery for some patients — even if no one is sure why.

The explanation of addiction transfer assumes that people who overeat are often predisposed to addiction. Researchers have reported addiction transfer in many forms — a recovering alcoholic might start chain-smoking, for instance. But the general idea is often contested, partly because it can be difficult to pinpoint the roots of addiction: some argue it is physiological, while others insist it is driven by psychology.

One physiological explanation for addiction, first described by a neuroscientist at the University of Florida named Kenneth Blum, is a blunted response to dopamine, a chemical that gets released in our brains when we perform high-reward activities such as eating, having sex, doing drugs and listening to music. Dopamine not only helps us register pleasure from these activities, it also motivates us to repeat them over and over again in search of more pleasure.

In 1990, Blum found a correlation between alcoholism and a genetic deficiency in dopamine-binding receptors in the brain, called D2 receptors. People with compromised D2 receptors seek higher thrills to satisfy their reward cravings than people with normal D2 receptors, Blum believes. He predicts that gastric bypass patients with a D2 deficiency turn to other high-reward activities, such as drinking alcohol, because they can’t binge eat with a constricted stomach.

Still, most scientists attribute addiction to a combination of genetic and environmental factors. Blaming addiction on a single gene is too simplistic, said Lance Dodes, a psychiatrist based in Boston who has written three books on the topic, including one called Breaking Addiction. Dodes believes addiction has a psychological basis. He argues that consuming a substance or behaving compulsively provides an outlet for people who feel otherwise unable to take direct action in their lives. Many of his alcoholic patients, he said, start feeling better the moment they decide to take a drink — not when the alcohol actually enters their bodies.

Addictions can be interchangeable because they are a psychological response to feeling trapped, said Dodes. “We call them separate addictions, but they’re really just one mechanism.”

But one major problem with Blum’s and Dodes’s ideas is that there’s little evidence of higher alcoholism rates after a different common bariatric surgery: gastric banding. Also known as lap banding, this surgery installs an inflatable belt around the stomach to constrict it. Unlike gastric bypass, banding does not permanently alter the stomach’s architecture.

The difference between the two surgeries suggests that alcohol abuse is related to structural changes from gastric bypass, said Alexis Conason, a New York City psychologist and researcher.

In 2012, Conason published a study in JAMA Surgery that found a significant increase in alcohol use for patients after gastric bypass, but not gastric banding. The study also found no significant increase in patients’ use of other drugs, including cigarettes, or compulsive behaviors such as gambling. “If it were addiction transfer, we’d be seeing it across the board,” Conason said.

Researchers have proposed a few physiological explanations for increased alcoholism specifically after gastric bypass. Some believe it’s due to changes in alcohol metabolism, since alcohol enters the bloodstream more quickly in a smaller stomach. A 2011 study from surgeons at Stanford University found that six months after surgery, gastric bypass patients reached higher blood alcohol levels more quickly than they did before surgery. This type of fast and high peak often characterizes addictive drugs, said North Dakota’s Mitchell. Cocaine and heroin, for example, both produce brief, intense rushes that leave users wanting more.

But it’s also possible that increased alcohol dependence has nothing to do with alcohol absorption in the stomach. Recently, a team of researchers led by neuroscientists at the Pennsylvania State University College of Medicine found that rats that had been given gastric bypasses developed a higher dependence on alcohol. Here’s the twist: the effect held even when the rats were given alcohol intravenously instead of orally. The authors concluded that alcohol abuse after gastric bypass could very well occur independently of how quickly alcohol passes from the gut to the bloodstream.

Instead, anatomical changes to the stomach might impact patients’ dopamine response, the Penn State researchers suggested. Some scientists have found that gastric bypass surgery can alter the signaling of D2 receptors. The mechanism for this is unclear, although preliminary research has identified altered patterns of gene expression in areas of the brain that process dopamine. Other researchers suggest that appetite-mediating gut hormones play a role, particularly those that affect dopamine signaling, such as insulin, leptin and grehlin. Scientists have shown that leptin and grehlin levels change after gastric bypass surgery, and both hormones are known to modulate alcohol consumption.

It’s also possible the explanation is not so clear-cut. Conason admits that researchers can’t completely write off addiction transfer, and there might be other reasons why alcoholism is more prevalent than other addictions. For instance, gastric bypass patients may simply be more likely to drink alcohol than take other drugs, which are less socially acceptable.

The risk of alcohol abuse is serious, Mitchell said, but it is one of many considerations for gastric bypass candidates. Particularly for people facing life-threatening conditions such as diabetes or heart disease, the possibility of alcohol abuse might not be strong enough for patients to actually forgo the surgery.

Even so, Mitchell and most of his colleagues agree that doctors need to strongly communicate the risk of alcohol abuse to patients before surgery. In many cases, including Jackie Kim’s, doctors don’t highlight the risk at all.

After years of counseling and seeing her addiction wreak havoc on her relationships and physical health, Kim reached a breaking point. She hasn’t touched a drink in more than two years. “But it wasn’t easy,” Kim said of her hard-fought recovery. She knows that others also might not have been coached on the risks, so she mentors patients who have had the surgery and posts in bariatric support groups online.

Kim believes that being aware of the problem would have made all the difference for her. “I experienced a lot of anger with my surgeons afterwards for not doing a better job educating me,” she said. “If they had told me not to drink, I wouldn’t have started in the first place — and it wouldn’t have spiraled into what it became.”

*Correction, Jan. 10, 2015:

A previous version of this story identified Kenneth Blum by the incorrect profession. He is a neuroscientist.