Vitamin C can help treat patients with severe cases of Covid-19, a Kiwi-led review has found.

But the Otago University researcher behind the study, just published in scientific journal Nutrients, stresses larger studies are needed to give more certainty - and that vitamin C shouldn't be seen as something that can prevent or cure coronavirus.

Its role in Covid-19 treatment has come under global focus since the pandemic began, with a slew of randomised clinical trials underway.

Otago's Associate Professor Anitra Carr has been monitoring studies, and said there were some promising indications vitamin C could be used as an "adjunctive" treatment - or one that complemented a main therapy.

She said groundwork had already been laid by decades of research into the role of vitamin C in the prevention and treatment of pneumonia and sepsis.

A 2017 study found high-dose IV vitamin C treatment - along with thiamine and corticosteroids - appeared to prevent deaths among people with sepsis, which an overwhelming response to infection causing dangerously low blood pressure and organ failure.

Another study published last year assessed the effect of high-dose vitamin C infusions among patients with severe infections who had sepsis and acute respiratory distress syndrome (ARDS), in which the lungs filled with fluid.

While that study's main outcome measures did not improve within the first four days of vitamin C therapy, there was a lower death rate at 28 days among treated patients.

Though neither of these studies looked at vitamin C use in patients with Covid-19, sepsis and ARDS are the most common conditions leading to ICU admission, ventilator support, or death among those with severe Covid-19 infections.

"A recent study that came out of the US showed that patients with coronavirus in ICU with Covid-19 also had very low vitamin C levels," Carr said.

"That's because the body chews through a lot more of it when you get an infection - and your requirements increase significantly.

"But the standard doses given in the intensive care unit aren't enough to compensate, given a severe case of the disease comes with a huge inflammatory response and oxidative stress."

By delivering vitamin C intravenously, she said, high doses could rapidly reach the body.

"That's critical, as all patients need these high doses quickly, because, at death's door, you need to slow down the disease as much as possible."

It could also give clinicians more time to act, she said.

Among preliminary data that had come out of a number of hospital trials around the world was that from a small study carried out in Wuhan, China, where the pandemic began.

"That showed that, in people who had the most severe cases of coronavirus disease - or with the most organ damage and failure - that group responded to the vitamin C, with fewer mortalities compared to those who received a placebo," she said.

"This is just the first study, and many more will come out over the next year or so. Each will add more pieces to the puzzle, but I suspect that it will help these patients, based on the previous work with pneumonia and sepsis."

Outside the clinical space, the pandemic has also prompted interest in vitamin C among consumers.

Here in New Zealand, sales of oranges, kiwifruit and supplements have surged - something the citrus industry has put down to Kiwis trying to stay healthy amid the Covid-19 crisis.

Some have wrongly turned to the vitamin as a Covid cure: on social media, misleading claims that a large intravenous dose of vitamin C can "stop Covid-19" have been spread widely.

While standard doses of vitamin C are generally harmless, taking high doses risked causing a number of side effects, such as an increased risk of kidney stones in people already susceptible to that condition.

Ultimately, there was no evidence that taking vitamin C could help prevent infection - but Carr said taking adequate amounts might benefit some people.

"Some research has shown that, if you're under enhanced stress, your risk for infection increases, so in those people vitamin C may decrease the risk of getting it," Carr said.

"But in the general everyday population, who aren't under extreme physical stress, it may not decrease your chances of catching the disease."

However, the vitamin has been shown to decrease the duration and severity of infections in the general population.

This could potentially decrease progression of an infection to more severe conditions such as sepsis that require ICU admission.