Reasons for Bad Metallic Coffee Breath

Coffee is a drink you probably rely on to feel awake and alert. Others may easily notice you're a coffee drinker, but it might not be because of your energy level. "Coffee breath" has become infamously associated with America's popular beverage. Different theories exist regarding the cause of that stale, "metallic" smell in your mouth, and some researchers believe it's not even caused by coffee at all.

Two women talk over a cup of coffee in a cafe. Credit: Design Pics/Design Pics/Getty Images

Acidic Environment

One of the more common theories about the cause of coffee breath is that coffee is acidic. Coffee creates an acidic environment in your mouth, which allows for the growth and reproduction of bacteria. This acidic environment is the reason for the metallic taste and smell that remains in your mouth after drinking coffee, Disabled World reports.

Dry Mouth

Coffee itself may not be the cause of bad breath, notes registered nurse Justyna Trzesniowksi on 891 ABC Adelaide. Instead, the real cause may be that the coffee was consumed on an empty stomach. Because coffee is a diuretic, it can cause your mouth to dry out if you drink it without any other food or water. This dry mouth encourages the growth of bacteria, Trzesniowski says. This effect is made worse when milk or cream in coffee is added to the bacteria-laden dry mouth, reports Tel Aviv University breath specialist Mel Rosenberg. This can actually cause a sort of fermentation of the milk and the bacteria, resulting in unpleasant odors.

Lactose Intolerance and Reflux

Lactose intolerance and preexisting high acid levels in the body may also contribute to coffee breath, according to Trzesniowski. People with lactose intolerance may have this bad breath reaction to coffee with milk, while others may experience indigestion or reflux -- a condition not known to freshen breath. If you are lactose intolerant, you may want to seek out a nondairy cream option.

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