The common belief is that coffee, being acidic and containing the drug caffeine may promote or exacerbate acid reflux, particularly as it is often cited by patients experiencing heartburn. However, the balance of research does not appear to support this.

An October 1975 study in the New England Journal of Medicine, Gastric acid secretion and lower-esophageal-sphincter pressure in response to coffee and caffeine, started from the premise that “Caffeine stimulates gastric acid secretion and reduces the competence of the lower esophageal sphincter in man. These effects of caffeine have been used as evidence that regular coffee should not be used by patients with peptic-ulcer disease or

gastroesophageal reflux.”

It used caffeine, normal coffee and decaffeinated coffee with test subjects: Sphincter pressure showed minimal changes in response to caffeine, but was significantly increased by both regular and decaffeinated coffee.

It concluded, "These data suggest that clinical recommendations based upon the known gastrointestinal effects of caffeine may bear little relation to the actual observed actions of coffee or decaffeinated coffee."

However, a December 1980 study in Gastroenterology, Inhibitory effect of coffee on lower esophageal sphincter pressure, found "coffee at either pH 4.5 or 7.0 caused a decrease in fasting and postcibal lower esophageal sphincter pressure in normal volunteers and patients with reflux esophagitis. The magnitude and the duration of the effect were greater after coffee at the lower pH. These data support the clinical belief that coffee may cause or aggravate heartburn by decreasing lower esophageal sphincter pressure."

A 1999 study Effect of coffee on gastro-oesophageal reflux in patients with reflux disease and healthy controls, published in European Journal of Gastroenterology & Hepatology, concluded, “Coffee has no important effect on gastro-oesophageal acid reflux in GORD patients, and no effect at all in healthy subjects.

Another 1999 study, Coffee and gastrointestinal function: facts and fiction. A review. published in the Scandinavian Journal of Gastroenterology, found, “Coffee promotes gastro-oesophageal reflux, but is not associated with dyspepsia. Coffee stimulates gallbladder contraction and colonic motor activity.”

A 2013 Cross-Sectional Study of 8,013 Healthy Subjects in Japan published in PloS one, No Association of Coffee Consumption with Gastric Ulcer, Duodenal Ulcer, Reflux Esophagitis, and Non-Erosive Reflux Disease. found “multivariate analysis of the healthy subjects could not detect significant association between coffee intake and upper gastroduodenal ulcer diseases. The meta-analysis including our present study was further conducted, which denied the meaningful association between them. We speculated that some preventive effects of coffee intake might outweigh the risks of increased gastric acid secretion: relaxing effect, antioxidant effect, phytochemical effect.”

A Korean study published in 2014, Association between coffee intake and gastroesophageal reflux disease: a meta-analysis. concluded, " there was no significant association between coffee intake and GERD."

A study published in Clinical Gastroenterology & Hepatology in December 2015, Coffee or Tea, Hot or Cold, Are not Associated With Risk of Barrett's Esophagus concluded "data do not support an association between consumption of coffee or tea and risk of BE (Barrett's Esophagus). It is unlikely that avoidance of coffee or tea will protect against BE."

An Italian study, Association between coffee or tea drinking and Barrett's esophagus or esophagitis, published in European Journal of Clinical Nutrition in May 2017 produced some confusing results appearing to show those who used to drink coffee but had now stopped were more at risk of developing Barrett’s: "Our data were suggestive of a

reduced risk of BE and E with tea intake. An adverse effect of coffee was found among BE patients who had stopped drinking coffee. Coffee or tea intakes could be indicative of other lifestyle habits with protective or adverse impact on esophageal mucosa."

In fact, other studies also showed coffee may actually have a beneficial role.

A study published in the Annals of Internal medicine in July 2017, Coffee Drinking and Mortality in 10 European Countries, concluded: Coffee drinking was associated with reduced risk for death from various causes. This relationship did not vary by country.

A similar study, Association of Coffee Consumption With Total and Cause-Specific Mortality Among Nonwhite Populations, published concurrently in the same journal, concluded: Higher consumption of coffee was associated with lower risk for death in African Americans, Japanese Americans, Latinos, and whites.

In a September 2019 review article in the Journal of Thoracic Disease, The role of diet in the development and management of gastroesophageal reflux disease: why we feel the burn? the authors noted, "early literature suggests coffee decreases LES tone after ingestion. But despite this transient effect, the most recently published meta-analysis showed no discernable association between coffee intake, GERD symptoms, or mucosal disease."

A widely publicised 2019 paper associating very hot coffee and oesophageal cancer showed a link between very hot drinks and Squamous Cell Carcinoma, a cancer of the throat and upper oesophagus that is not related to acid reflux. A prospective study of tea drinking temperature and risk of esophageal squamous cell carcinoma: "drinking 700 mL/day or more at a higher‐temperature (≥60°C) was consistently associated with an about 90% increase in ESCC risk."

From Can Coffee Cause Cancer? The Research Says It’s Highly Unlikely

"In the spring of 2018, a California court launched a firestorm when it ruled that coffee sold within the state may need a cancer warning label due to the presence of a chemical called acrylamide, a potential carcinogen.

"In 2016, a working group of the International Agency for Research on Cancer (IARC) for the World Health Organization (WHO) evaluated if drinking coffee could cause cancer.

"After reviewing over 1,000 studies, they concluded that there wasn’t conclusive evidence to classify coffee as carcinogenic. In fact, they found that many studies indicated no effect of coffee consumption on the development of pancreatic, prostate, and breast cancers.

"studies have found no association between dietary acrylamide intake and risk for several cancers"

This paper published in January 2022 may have been authored by Astrid Nehlig ''a scientific consultant at the Institute for Scientific Information on Coffee ... but had no role in the organization or preparation of the manuscript", but is pretty comprehensive looking at Effects of Coffee on the Gastro-Intestinal Tract concluding, "Coffee consumption has not been reported to generate any deleterious effects on the various organs of the digestive tract,"