Individuals with known allergy or hypersensitivity to barley flour or beer should avoid barley products. Severe allergic reactions (anaphylaxis) and skin rashes have been reported from drinking beer made with malted barley. Patients with allergy/hypersensitivity to grass pollens, rice, rye, oats or wheat may also react to barley.

"Bakers' asthma" is an allergic response from breathing in cereal flours among workers of the baking and milling industries, and can occur due to barley flour exposure. If an individual is allergic to one cereal (like barley), there is a possibility that other cereals may cause similar symptoms.

Barley appears to be well tolerated in non-allergic, healthy adults in recommended doses for short periods of time, as a cereal or in the form of beer. Individuals with celiac disease (wheat allergy) may have a higher tendency to develop gastrointestinal (stomach) upset with barley products. Barley may cause a feeling of "fullness."

Theoretically, eating large amounts of barley may lower blood sugar levels. Caution is advised in patients with diabetes or hypoglycemia, and in those taking drugs, herbs, or supplements that affect blood sugar. Serum glucose levels may need to be monitored by a healthcare provider, and medication adjustments may be necessary. Hordenine, a chemical in the root of developing barley, may stimulate the sympathetic nervous system. The effects of hordenine from barley in humans are not clear, although theoretically increased heart rate or wakefulness may occur.

Eye, nasal, and sinus irritation or asthmatic reactions can occur from exposure to barley dust. Some individuals may experience inflammation or irritation of the skin, eyelids, arms or legs. Contact with the malt in beer may cause skin rash.

Traditionally, women have been advised against eating large amounts of barley sprouts during pregnancy. Infants fed with a formula containing barley water, whole milk, and corn syrup developed malnutrition and anemia, possibly due to vitamin deficiencies.

For constipation, limited research has used 9 grams of germinated barley foodstuff (GBF) daily for up to 20 days. For coronary heart disease (CHD), the FDA is authorizing food manufacturers to use the health claim for barley and the reduced risk of CHD. To qualify for the health claim, the barley-containing foods must provide at least 0.75 gram of soluble fiber per serving of the food.

For high cholesterol, 1.5 milliliters of barley oil twice daily or 30 grams of barley bran flour daily by mouth has been used in studies. Also, for ulcerative colitis (mild-to-moderate), germinated barley foodstuff (GBF) 10 grams taken three times daily has been studied and reported as well tolerated.

There is not enough scientific information to recommend barley for use in children.

Fiber in barley may decrease the absorption of medications taken by mouth and prevent full beneficial effects. Eating barley in large quantities may lower blood sugar concentrations. Caution is advised when using medications that may also lower blood sugar. Patients taking drugs for diabetes by mouth or insulin should be monitored closely by a qualified healthcare professional. Medication adjustments may be necessary.

Barley has been associated with decreased total cholesterol and low-density lipoprotein (LDL) concentrations, and may act additively with other cholesterol-lowering agents. Hordenine, a chemical in the root of the developing barley, stimulates the sympathetic nervous system. In theory, taking hordenine with stimulant drugs may result in additive effects such as increased heart rate or wakefulness.

Fiber in barley may reduce the absorption of some herbs and supplements that are taken by mouth. In theory, barley may lower blood sugar levels. Caution is advised when using herbs or supplements that may also lower blood sugar. Blood glucose levels may require monitoring, and doses may need adjustment.

Barley has been associated with decreased total cholesterol and low-density lipoprotein (LDL) concentrations, and may add to the effects of cholesterol-lowering agents. Hordenine, a chemical in the root of the developing barley, stimulates the sympathetic nervous system. In theory, taking hordenine with stimulant agents such as ephedra or caffeine may result in additive effects such as increased heart rate or wakefulness.