A daily tablet used to treat arthritis and Crohn's disease could also reverse hair loss, a US study has found.

After six months on upadacitinib, more than half of previously bald patients regrew almost an entire head of hair. 

The drug, which costs the NHS roughly £28 a pill, works by stopping the immune system from attacking hair follicles - which is the cause of baldness in the condition alopecia areata.

The disorder, which affects around 400,000 Britons, can develop at any age. 

High profile patients include TV presenter Gail Porter, who, in 2005, lost all her hair within four weeks. 

This is a different condition from pattern baldness which affects around 85 per cent of men to some degree before the age of 50, and around 50 per cent of women by 65.

Other arthritis drugs, such as baricitinib, have already been approved in the UK for the treatment of alopecia areata.

Now researchers say upadacitinib should be considered as a potential treatment for adults and adolescents with severe hair loss. 

'Alopecia areata has a profound psychosocial burden on patients, but conventional off-label treatments have variable and limited efficacy,' Dr Arash Mostaghimi, professor of dermatology and the study's lead author, explained.

Even with the currently approved treatments, experts say there 'remains a clear unmet need for additional and improved therapies.' 

Some patients with the condition see their hair fall out in small patches, but others can lose all their body hair within a matter of weeks. 

The new findings are based on two international trials led by dermatologists at Brigham and Women's Hospital, involving nearly 1,400 patients aged 12 to 64 with severe alopecia areata. 

All participants had hair loss that had persisted for up to eight years, with no signs of spontaneous hair regrowth in the past six months. 

Participants were randomly assigned to receive either 15mg or 30mg of upadacitinib a day, or a placebo. 

After 24 weeks, around 45 per cent of those taking the lower dose and 55 per cent on the higher dose had achieved at least 80 per cent hair coverage on their scalp, compared with just 3.4 per cent of those given a placebo. 

The majority of patients experienced noticeable differences within two months of starting treatment. 

Around one in five patients taking the higher dose achieved complete hair regrowth after six months.

Others also saw improvements in eyebrow and eyelash growth and reported a better quality of life.

The most common side-effects reported were acne, upper respiratory infections and cold-like symptoms. 

But no new safety concerns were identified during the trial. 

Writing in the journal JAMA Dermatology, the team concluded: 'Upadacitinib may be an effective therapeutic option for adults and adolescents with severe alopecia areata.' 

In arthritis, the drug - which belongs to a class of medicines called JAK inhibitors - works by stopping the immune system from attacking the joints. 

 Alopecia is a general term for hair loss, and there are at least ten different types, from pattern balding to chemo-induced hair loss. 

Alopecia areata is an autoimmune disease. While it can affect men and women of any age, around half of cases start in childhood.

Experts do not know exactly what triggers the condition, but the British Association of Dermatologists lists previous infections and stress as possible triggers. 

However, it is also possible that these links may be coincidental. 

It has also been linked to thyroid disease, vitiligo and type 1 diabetes. 

Currently, there is no cure for the condition but, depending on the extent of hair loss, for around four in five patients there is a good chance that regrowth will occur within a year without treatment.  

Those with more than half the hair lost at the beginning, or with complete hair loss at any stage, have only about a one in 10 chance of full recovery.