‘Take on an Empty Stomach.’ How Do You Know When Your Stomach Is Empty?

Q. Many medications should be taken on an empty stomach. How do you know when your stomach is empty?

A. Two hours after eating is a crude rule of thumb. A more accurate answer depends on the drugs you are taking and your medical conditions.

Doctors began studying gastric emptying times in the 1940s at Guy’s Hospital in London. Gastric emptying time is the length of time required for the stomach to return to empty after a meal.

In 1951, they published findings on 21 volunteers. Each subject ingested a test meal of gruel. Then, the contents of their stomachs were sucked back out. By repeating the experiment at various time points, the doctors were able to determine how long it took for their stomachs to empty. From 190 experiments, the doctors calculated the average emptying time to be about two-and-a-quarter hours.

In 1966, other investigators began using radioactive substances to measure gastric emptying. With refinements over the years, this technique has become the standard for assessing the rate of gastric emptying.

Standards for such testing have been set by the American Neurogastroenterology and Motility Society and the Society of Nuclear Medicine. They state that a normal stomach should be 90 percent empty after four hours. The difference between this standard and the earlier study probably reflects differences in foods. Solids take longer to digest than liquids; fats take longer to digest than protein or carbohydrates.

The Food and Drug Administration incorporates these data, standards and thousands of drug-food studies into actionable recommendations. The F.D.A. defines an empty stomach as “one hour before eating, or two hours after eating.” The F.D.A.’s two-hour rule is just a rule of thumb; the stomach will probably not be completely empty.

The specific definition of an empty stomach varies from drug to drug. The popular anti-osteoporosis drug Fosamax, for example, should be taken “at least one-half hour before the first food, beverage, or medication of the day.” This promotes its absorption and reflects the need to stay upright after taking Fosamax, which lessens its potential to cause esophageal irritation. The same recommendation applies to the related drug Actonel, but a full hour is recommended for the related drug Boniva.

Similarly, the thyroid drug levothyroxine should be taken “on an empty stomach, one-half to one hour before breakfast.” But one should not assume that an empty stomach always implies that a medication should be taken first thing in the morning. Some medications should be taken on an empty stomach at bedtime. This is the case with the H.I.V. drug Sustiva. Taking Sustiva on an empty stomach regulates its absorption. And, taking it at bedtime makes the dizziness and drowsiness that it causes more tolerable.

Authoritative guidance for every drug can be found in its package insert, which contains each drug’s F.D.A.-approved prescribing information. It is often found glued to the drug package in the form of a tightly folded, fine print flyer. The insert is also available online at DailyMed.

There are additional individual factors to consider. Some medical conditions, such as diabetes, can delay gastric emptying. Others can accelerate gastric emptying, such as certain types of bariatric surgery. As always, your doctor should be your guide to what constitutes an empty stomach for you.