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Why can I taste saline and medications when they're inserted into my IV?

It is quite common for patients to complain of a funny taste in their mouth when their IV lines are flushed with prepared syringes of saline. Who knew?

Someone recently asked me if I had ever heard a patient complain that they could taste saline and medications when injected into their IV. Truth is, I had never heard of this before, but apparently it is quite common for patients to complain of a funny taste in their mouth when their IV lines are flushed with pre-prepared syringes of saline in order to keep them open and avoid clotting.

Why this occurs is an interesting question. We first though need to explain how taste works. We essentially have four different types of taste buds that respond to sweetness, sourness, bitterness and saltiness. There is same debate about whether we have fifth type of taste bud for savory tastes (sometimes also referred to as umami). The receptors on these taste buds are stimulated by glutamate, which some might remember is added to foods in the form of monosodium glutamate or MSG. Students of recent history will remember how much panic there was surrounding MSG for a while. But time, science and level-headedness eventually prevailed and this particular health scare has probably faded into memory for most.

The astute reader will realize that sweet, sourness, bitter, salty, and savory do not fully account for the rich diversity that we understand as the taste of our food. That is because most of what we consider as “taste” is not actually mediated by our taste buds. Much of the “taste” of our food is mediated by the texture and the smell of what we eat. When we say that a cookie tastes stale, what we really mean is that the texture is hard and no longer moist or chewy. When we say something tastes soggy, what we really mean that it is water-logged and possibly under cooked.

While texture is indeed important, it is our sense of smell that accounts for a significant degree of the richness and diversity of the “taste” of our food. We have millions of odour receptors in our nose, compared to hundreds of taste buds in our mouth. Case in point, when you have a cold and your nose is blocked, food tastes much more bland and unappetizing. Although we often neglect the importance of our sense of smell, the world would be a far more banal place without it.

So now we come to our original question. Can you taste something injected into your IV? Now of course, many medications can give you a strange taste in your mouth, which is termed dysgueusia. Medications taken orally can be directly tasted and medications given intravenously stream can be excreted into the saliva and tasted that way. Some medications do not have a taste in and of themselves but can disrupt your sense of taste or smell, and thus make food taste “different.”

However, the current question referred to a very specific phenomenon of patients developing a strange taste in their mouth just as, or shortly after, fluid is injected into their IV. This fluid is generally a saline flush, a solution of salt water designed to flush the catheter and keep it from clogging.

The key here is that medical personnel are not injecting pure water into IV lines. One never finds pure water in nature because all drinkable water has dissolved minerals in it. In medicine though, while pure water exists, you cannot inject it directly into IV lines. If you did, the water would rush into red blood cells because of osmosis. Osmosis is the movement of water from a dilute environment to a more concentrated one. So, pure water injected into an IV would diffuse via osmosis into the more concentrated environment inside red blood cells, make them swell up because of the excess water and eventually make them burst. To avoid this, IV solutions are almost always saline solutions, essentially solutions of salt and water.

Of course, these solutions do not contain just salt and water. They contain microscopic amounts of other molecules also dissolved in the water. The prevailing theory is that these microscopic molecules dissolved in water, once they enter the blood stream, are then excreted into the lungs where they are breathed out of the body. While our exhaled breath is mostly nitrogen, oxygen and carbon dioxide, it also contains trace amounts of other gases, of which some are likely the gaseous forms of these previously dissolved molecules.

Many people are blissfully unaware of their own bad breath, but even the most un-self-aware individual would smell a new molecular component in the air they exhale.

Patients are likely able to identify these new scents in their exhaled breaths. While it is commonly believed that we humans have a bad sense of smell, in fact our sense of smell is quite good (remember it accounts for the majority of what we think of as taste). Thus, people who can “taste” when their IV lines are flushed with saline, are likely not so much tasting it as they are smelling it.