Anosmia (Loss of Smell)

How smell works

Odor molecules travel into the top of the nasal cavity, either through the nostrils or from the back of the mouth when you eat. They bind to receptors on olfactory nerve cells, which send signals through tiny holes in the skull to the olfactory bulb in the brain. Smell can be lost at any point:

• Conductive loss: odors cannot reach the receptors because the nose is blocked (swelling, polyps).
• Sensorineural loss: the receptors or nerves are damaged (viruses, head injury, toxins).
• Central loss: the brain's smell pathways are affected (neurodegenerative disease, tumors).

Causes of anosmia

Post-viral and COVID-19 smell loss

Viruses can damage the supporting cells and nerve cells of the smell lining. During the early COVID-19 pandemic, sudden loss of smell was one of the most distinctive symptoms. Most people recover within weeks, but a minority have lasting reduction. During recovery, many experience parosmia, where familiar smells such as coffee, onions or meat smell rotten or chemical. This is usually a sign that nerves are regenerating.

Effects of losing smell

• food tastes bland, because flavor relies mostly on smell (taste itself detects only sweet, salty, sour, bitter and umami)
• reduced appetite or, conversely, over-seasoning and overeating
• safety risks: not detecting smoke, gas leaks or spoiled food
• reduced enjoyment and quality of life, and sometimes depression or anxiety

Diagnosis

An ENT (ear, nose and throat) specialist will ask about onset, infections, injuries, medicines, smoking and other symptoms, and:

• examine the nose with a thin camera (nasal endoscopy) to look for polyps or swelling
• perform smell testing, such as the University of Pennsylvania Smell Identification Test (a scratch-and-sniff booklet) or "Sniffin' Sticks"
• order a CT scan of the sinuses if sinus disease is suspected
• order an MRI of the brain if there is no obvious cause, especially with neurological symptoms

Treatment

• Sinus disease and polyps: steroid nasal sprays or rinses, short courses of steroid tablets, sinus surgery, or biologic medicines for severe nasal polyps. Smell often improves.
• Allergies: avoiding triggers, antihistamines and steroid sprays.
• Post-viral or post-traumatic loss: olfactory (smell) training: sniff 4 strong scents, traditionally rose, lemon, clove and eucalyptus, for about 20 seconds each, twice a day, for at least 3 to 6 months. It is the best-supported treatment and can improve recovery.
• Stop or switch a medicine that may be responsible, under medical advice.
• Stop smoking.

There is no proven medicine to restore post-viral smell loss, though some treatments are being studied.

Living with anosmia

• install smoke and natural gas detectors and test them regularly
• label food with dates and use them rather than smell to judge freshness
• ask someone to check for gas leaks, smoke or body odor if you are concerned
• enhance meals with texture, temperature, color and spices that stimulate the trigeminal nerve, such as chili and mint

Outlook

Recovery depends on the cause. Smell loss from congestion usually returns once the nose is cleared. After viral infections, many people improve over 6 to 12 months, and some recover up to 2 years later. Recovery after head injury is less common but possible. Smell loss from aging or neurodegenerative disease is usually permanent.

Frequently asked questions

What causes anosmia?

The most common causes are viral infections including colds and COVID-19, chronic sinusitis and nasal polyps, and head injuries. Aging, smoking, some medicines, and neurological diseases such as Parkinson's and Alzheimer's are other causes. Some people are born without a sense of smell.

How long does anosmia last?

It depends on the cause. Loss from a cold or allergy usually returns within days to weeks. Post-viral loss, including after COVID-19, often improves within months, though some people take a year or longer. Smell training may help speed recovery.

Does smell training really work?

Studies show that smell training, sniffing a set of strong scents twice daily for several months, improves smell recovery in post-viral and post-traumatic loss compared with no training. It is safe, inexpensive and recommended by ENT specialists.

Is loss of smell a sign of a serious disease?

Usually it is caused by infection or sinus disease. Occasionally, gradual smell loss can be an early sign of Parkinson's or Alzheimer's disease, and rarely a tumor. A doctor should assess loss of smell that persists without a clear cause.

Why does food taste different when I lose my sense of smell?

Most of what we call flavor comes from smell, as aromas travel from the mouth to the back of the nose. Taste buds alone detect only sweet, salty, sour, bitter and umami. Without smell, food often tastes bland.