Americans are suffering a bad case of indigestion.

Our discomfort has fed a market for heartburn medications that topped $13 billion in sales last year.

Some point a finger at drugmakers’ relentless pitches. Prilosec, Nexium, Prevacid and other drugs are heavily advertised on television and in magazines.

But doctors say they’ve seen a startling jump in actual diagnoses.

“I’ve been in this business for 40 years. I’m pretty sure I wasn’t blind and missing it the first 25 years. There’s been a real increase,” says Dr. H. Worth Boyce, director of the Joy McCann Culverhouse Center for Swallowing Disorders in Tampa, Fla. “It’s near the top of gastrointestinal disorders.”

Heartburn is a symptom. The medical problem is called GERD, for gastroesophageal reflux disease. Something gastric (from the stomach) backs up (refluxes) into the esophagus, says Dr. Paulo Pacheco, co-author of “Living with Chronic Heartburn” (Healthy Living Books, $15.95).

Reflux happens because a sphincter, or muscle, at the base of the esophagus malfunctions.

GERD likely plagued the caveman who ate too much mastodon, Pacheco writes. But the disorder has garnered much more attention during the past 15 years as the medical community has recognized its potential seriousness and pharmaceutical companies have found drugs to treat it.

Modern man suffers more because of lifestyle, most experts say. Greasy fast food, alcohol and tobacco use, and poor eating habits, from overeating at one sitting to obesity, aggravate GERD symptoms.

“We don’t really know why there are so many more cases,” he says.

Almost everyone suffers symptoms occasionally: chest pain, a burning sensation, nausea, sour taste in the mouth, burping, hiccups, cough, throat soreness.

Severe or repeated symptoms indicate GERD. More than 21 million Americans have the condition. More than 2 million a year go to the emergency room with chest pain they fear is a heart attack. A small percentage develop cancer of the esophagus.

“A lot of the thinking about reflux has changed,” says Dr. G. Richard Locke, director of the esophageal group at the Mayo Clinic in Rochester, Minn. “It used to be people would get symptoms and take an antacid, and lots of them. Doctors used to think it was a short-term condition.”

Today reflux is treated as a chronic condition, and a daily pill is used to prevent symptoms before they start.

The biggest players in the pharmaceutical battle are proton-pump inhibitors, or PPIs, which block acid production in the stomach. All brands work in similar ways. But direct-to-consumer marketing will continue its onslaught as drugmakers try to distinguish their generics and brand names.

Consumers often pay more than $100 a month for GERD medications.

To hold overall drug costs down, the Food and Drug Administration recently announced that it wants to increase by 50 percent switches from prescription to over-the-counter versions of popular drugs.

Typically, drug manufacturers make the switch when a drug’s patent expires, developing a prescription drug to take its place. The industry contends that FDA-forced switches will remove incentive for research and development, says the Tufts Center for the Study of Drug Development in an impact report.

Some caregivers fear the move to over-the-counter will put patients at risk.

“This is very scary because people will go purchase drugs and they may not have what they think they have,” says Bobbi Sheffield, clinical dietitian at Helen Ellis Memorial Hospital in Tarpon Springs, Fla. “They need to be diagnosed by a physician and supervised by a physician.”

Boyce also urges GERD sufferers to see a doctor first.