Researchers in the United Kingdom are testing to see whether vitamin A can be used to restore the sense of smell in people who lost it because of COVID-19.

A loss of the sense of smell is a common side-effect of COVID-19, with around eight people in 10 who have a symptomatic infection experiencing some reduction in their ability to smell.

For some patients-around 18 percent according to one study-the symptom is severe, causing anosmia, or total smell loss.

This sensory reduction may last a few weeks or persist for far longer and some patients who were infected early in the pandemic report that their sense of smell has still not returned to normal.

Previous research in Germany has shown that vitamin A nasal drops may help repair olfactory damage caused by viral infections, prompting scientists at the University of East Anglia to use the treatment on COVID-19 patients in a new UK government-funded study.

"Around one in 10 people who experience smell loss as a result of COVID-19 report that their sense of smell has not returned to normal four weeks after falling ill," said Carl Philpott, a professor of olfactology at the university.

Philpott said people with smell loss can experience depression, anxiety, feelings of isolation, and weight loss due to reduced appetite. He said other risks include failure to identify hazards, such as gas or spoiled foods.

"A key problem for patients and their clinicians is the lack of proven effective treatments," Philpott said.

Prior to the pandemic, researchers at the Interdisciplinary Center for Smell and Taste in Dresden, Germany, established a link between vitamin A drops and improved outcomes in patients with viral-induced smell loss.

Vitamin A is known to play a decisive role in the generation of olfactory receptor neurons, leading scientists to theorize that the drops can help repair damaged nasal tissue.

"We want to find out whether there is an increase in the size and activity of damaged smell pathways in patients' brains when they are treated with vitamin A nasal drops," said Philpott. "This would show recovery of the damage caused by common viral infections, including COVID-19, in the nose."

The University of East Anglia team says around 5 percent of the general population live with some form of smell loss, and the rise of COVID-19 means millions more are now affected, creating an "unprecedented worldwide demand for treatment".

In the new study, 38 patients will receive a 12-week course of nasal vitamin A drops and 19 will receive inactive peanut oil drops. Before and after the treatment course, researchers will use MRI scanning to see if there is a change in the size of the olfactory bulb, an area above the nose where the smell nerves join together and connect to the brain.

They will also test for neural activity and olfactory performance by introducing distinctive odors, such as roses or rotting eggs, while patients are undergoing MRI imaging.

The UK National Institute for Health Research has awarded 250,400 pounds ($337,200) to help fund the study.