A sugar substitute found in thousands of candies, desserts and diet drinks could promote hair growth in people suffering from baldness, a study suggests.

Stevia, a natural, zero-calorie sweetener used in more than 40,000 products worldwide, contains compounds known as steviosides, which give it its sweet flavor.

Now, Australian researchers have found that if these compounds are mixed with the popular over-the-counter hair loss drug minoxidil, commonly known as Rogaine, it may help revive hair growth on the head.

Minoxidil is a popular and cheap hair loss treatment in the US with an estimated 40million Americans using the drug every year. 

In tests on mice, scientists placed a microneedle patch infused with a mix of minoxidil and steviosides on the rodents' bald back every day for a month and found it triggered new hair growth across 67.5 percent of their back by the experiment's end.

Mice who had the standard two percent minoxidil solution, without steviosides, sprayed onto their backs every day for the same period saw regrowth over just 25 percent of the bald area.

Dr Lifeng Kang, a pharmacist at the University of Sydney who led the research, said: 'Using stevioside to enhance minoxidil delivery represents a promising step toward more effective and natural treatments for hair loss, potentially benefitting millions worldwide.'

Researchers aren't certain why stevia extracts may boost hair growth, but they suggest it could be because they help the body absorb more of the minoxidil, increasing the effectiveness of the medication.

Minoxidil works by stimulating blood flow to hair follicles on the scalp, prolonging their growth phase and encouraging new strands of hair to grow.

It is commonly applied topically as a gel or foam, but because the skin absorbs it poorly, users must apply the drug twice a day, often for six months or more, before seeing results.

It is safe to use, but in rare cases the drug can cause side effects including swelling in the hands and feet, chest pain, increased face and body hair growth and nausea and vomiting. 

The drug can also be taken orally, with around 95 percent absorbed by the bloodstream. Some patients, however, avoid this option over concerns it may affect hair growth elsewhere on the body or cause more side effects. 

Several small studies have suggested patients using oral minoxidil are about twice as likely to experience the side effect hypertrichosis, or unwanted hair growth on areas of the body besides the head, compared to those who use the topical version.

Studies have shown that the oral and topical versions of the drug are equally effective, although some papers suggest the oral version may be more effective because more of the drug is absorbed by the body. 

There are no clear estimates for what proportion of patients use the pill form vs the gel or foam.

Researchers have tried to boost absorption of topical minoxidil in previous cases, adding alcohol and propylene glycol, an odorless fluid used in some drugs, to the medication, but this has led to side effects such as itching, rash and dandruff.

In the new study, published in the journal Advanced Healthcare Materials, the team used mice that had a bald back and split them into four treatment groups.

The Australian researchers split the mice in four groups: no drug; two percent MXD alcoholic solution, which had minoxidil mixed into an alcoholic solution; blank MN, which had a microneedle patch containing no medication; and SDP MN, which had a microneedle patch with minoxidil and steviosides.

A microneedle patch has previously been shown to also help reverse hair loss because it can boost blood flow to bald areas, causing some hair follicles to regrow.

In the group that received the microneedle patch with minoxidil and steviosides, the team used a solution of two percent minoxidil. The study did not specify how much steviosides mice received.  

Mice treated with either of the microneedle patches received their treatment once a day, while those using the two percent minoxidil were sprayed once a day.

After 35 days, scientists found virtually no change in hair cover in the mice that received no treatment and a microneedle patch only.

In the two percent minoxidil group, the hair that regrew was described as 'bluish-black patches' that started to appear on the skin by day 14. 

Researchers said the growth was 'slow' overall.

In the group that received minoxidil and steviosides via a microneedle patch, researchers said a 'large area of new hair growth had appeared by day 14'.

They added that the area 'increased rapidly', and covered a 'large area' of the mouse's back by day 35.

No side effects or adverse events were recorded. 

Further research is required to confirm the findings, the researchers said.

The study was small and only carried out in mice, with experts saying it is not clear whether stevia extracts mixed with minoxidil will have the same effect in humans.

Consuming more products containing stevia is not likely to prompt extra hair growth.

Stevia extracts bought in grocery stores normally contain steviosides alongside other agents to bulk out the mixture including erythritol, a sugar alcohol used to add a cooling sweetnes.

It comes amid a major breakthrough in hair loss treatments with a new medication, called PP405, that triggered hair regrowth within eight weeks, rather than the around six months that minoxidil and finasteride take to have an impact.

In the phase 2a trial, participants rubbed gel containing the medication onto their scalp daily and saw hair begin to regrow by week eight.

Overall, 31 percent of those who used the medication had a greater than 20 percent surge in hair density. In the placebo group, none saw any hair regrowth.

Stevia is used in many diet sodas including Coca-Cola Zero Sugar, Zevia and zero-sugar water drops used to add flavoring.

It is also in ice creams, such as several made by Halo Top, and a wealth of protein bars, including KIND zero sugar and some by Aloha and Luna, as well as being available as a sugar alternative to be mixed into coffees and teas.

Stevia is largely thought to be safe to use, but animal studies have suggested it could cause an imbalance in the gut microbiome, raising the risk of immune system problems. Some papers have also linked its use to depression.

Estimates suggest that by age 35 years about two thirds of men will experience some form of hair loss, with this number rising to 85 percent by the age of 50 years.

Among women, more than 50 percent will experience notable hair loss at some point during their lives, estimates suggest.