What causes loss of smell and taste?

Your ability to smell and taste are closely linked as taste and smell information are carried to the same part of your brain where they are processed together to create a complete sensory experience. Consequently, when you lose your sense of smell, your ability to taste is affected, and vice versa. 

Here, we’ll take a look at common causes for losing the ability to smell (anosmia) and losing the ability to taste (ageusia), and how these are treated. But first, it helps to understand how your brain perceives taste and smell. 

How do smell and taste work?

How smell works

Your ability to smell depends on a pathway of cells that run from your nose to your brain. 

It starts in the cells that line your nose, which have small hairs on them (cilia) that detect smells. This smell information is then transported along your olfactory nerves to your olfactory bulb, a structure that sits at the back of your nose. 

Next, these olfactory nerves connect to olfactory fibres in the olfactory bulb, which carry smell information onward to your brain, passing through the base of your skull via a spongy plate called the cribriform plate. 

How taste works

Taste similarly depends on a pathway of cells that lead to your brain. 

It starts with taste buds on your tongue; there are different taste buds for different types of taste. Sweet taste buds reside at the tip of your tongue, salty taste buds on the front sides, sour taste buds on the back sides, bitter taste buds on the back third of your tongue and umami (savoury) taste buds across your entire tongue. 

There are also taste buds in other parts of your throat, including your soft palate, epiglottis and pharynx.

Taste information is detected by your taste buds and travels along two nerves called cranial nerve VII and cranial nerve IX. These nerves carry taste information to your brain stem (the base of your brain), where they connect to other nerves that travel onward to the same part of the brain where smell is processed. 

Taste and smell information are both processed in a part of the brain called the orbitofrontal cortex. 

Causes of loss of smell

Loss of smell can be due to conductive or sensorineural problems. 

Conductive problems refer to conditions that block smell molecules from reaching cells that detect smell. This includes conditions such as nasal polyps and severe cases of a deviated nasal septum. Nasal polyps can make you feel as if your nose is blocked, affecting your sense of smell and taste, and producing discharge from your nose (a runny nose). 

Sensorineural problems refer to conditions that interfere with the transport of smell information to the brain. This includes conditions such as past head trauma that affects olfactory nerves and viral infections that affect your sinuses (eg the common cold, flu and COVID-19). A sinus infection can cause excessive discharge from your nose, facial pain, a general feeling of unwellness and fatigue. 

Smoking can also reduce your ability to smell, as can neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease. 

In rare cases, individuals are born without the ability to smell — this is called Kallmann syndrome.

Causes of loss of taste

Loss of smell often causes loss of taste, which is why conditions that cause loss of smell, such as nasal polyps and viral infections, can also be responsible for loss of taste. 

Other causes of loss of taste include smoking, diabetes, Sjögren’s syndrome, infection of your oral cavity (the space inside your mouth) and certain treatments, such as radiotherapy and particular medications (eg antidepressants such as amitriptyline). 

Nutrient deficiencies can also cause loss of smell, such as deficiencies in zinc, copper and nickel. 

Diagnosing loss of smell and/or taste

If you’re experiencing a loss or reduction in your ability to smell or taste, see your GP. They will ask you about your symptoms and medical history and may perform a physical examination. If appropriate, they may refer you to a specialist, namely an ENT surgeon.  

Your ENT surgeon may recommend that you have a nasal endoscopy. This is where a thin, flexible telescope-like camera with a light on the end is passed into your nose to look for structural causes of conductive loss of smell. 

Occasionally, if your nasal endoscopy doesn’t reveal any conductive causes for your loss of smell and/or taste, your doctor may recommend an MRI scan. If a conductive cause is identified and surgery is needed, you will also have a CT scan. 

If no structural abnormalities are found and you have no other nasal symptoms, the cause of your loss of smell and/or taste is likely to be sensorineural. 

Treating loss of smell and/or taste

Treating loss of smell

Conductive causes of loss of smell can usually be treated via surgery, for example, a nasal polypectomy to remove a nasal polyp, surgery to correct a deviated septum or sinus surgery to treat chronic (long-term) sinusitis. 

Sensorineural causes of loss of smell can sometimes be treated with steroids; however, if steroids aren’t effective, your doctor may recommend smell retraining therapy, which involves repeatedly presenting different smells to your nose to establish memories of those smells. 

It is important to note that if you have a loss of smell, you must ensure your home is properly fitted with gas and smoke detectors. 

Treating loss of taste

After assessing your symptoms, taking a medical history and a thorough examination, your doctor will check that the nerve pathways that carry taste information to your brain are working properly. 

Once an underlying cause has been identified, treatment will focus on alleviating the symptoms of that particular condition. 

For example, if a blood test detects nutritional deficiencies, your doctor may prescribe supplements. If radiotherapy is causing your loss of taste, your doctor may recommend an artificial saliva spray.