Image copyright Reuters Image caption Twelve-year-old Billy Caldwell - pictured with his mother Charlotte - has severe epilepsy

There is a need to review how cases for the medical use of cannabis are handled, the prime minister has said.

Theresa May made the comment after a special licence was granted allowing Billy Caldwell, 12, to be treated with cannabis oil for his severe epilepsy.

His mother tried to bring the banned substance into the UK last week but it was confiscated at Heathrow Airport.

He has been discharged from a hospital in London, having been admitted on Friday as his seizures intensified.

Charlotte Caldwell said what her son had endured was "horrendous" after he was left without access to the cannabis oil.

On Saturday, Home Secretary Sajid Javid approved the return of some of the confiscated oil after doctors at the hospital where Billy was being treated at made it clear it was a medical emergency.

Cannabis oil was first used to control Billy Caldwell's seizures in 2016.

It contains a substance called Tetrahydrocannabinol (THC), which is illegal in the UK but available elsewhere.

'Not dragging feet'

Ms Caldwell, from Castlederg in County Tyrone, said it was "vital" for families in the UK to be given "immediate access" to medicinal cannabis in cases where children needed it to treat a health condition.

"The fact that Billy has been discharged and is now with me is testament to the effectiveness of the treatment," she added.

Ms Caldwell demanded a change in the law, saying that "never again" should someone in her son's condition "be exposed to Home Office paperwork instead of medical treatment".

Health Secretary Jeremy Hunt said it was obvious the government was not "getting the law on this kind of thing right" and suggested a review would take place "as quickly as possible".

"We will do something; we'll do it as quickly as we can," he told BBC Radio 4's Today programme.

"The Home Office are not dragging their feet on this - the home secretary has said he will review this issue.

"It does take time because we've got to not only look at the law, we've got to look at the clinical evidence - we've got to make sure there are no unintended consequences."

Niamh Eastwood, the director of Release, a drugs law advice charity, said the legislation was an "absolute mess".

"It's not in the public interest for police to be chasing minor amounts of THC," she added.

"The law has created this mess - people are suffering and there are solutions to it."

Does cannabis have medicinal benefits?

Cannabidiol (CBD) and Tetrahydrocannabinol (THC) are two types of cannabinoids found naturally in the resin of the marijuana plant.

A cannabis-based drug called Sativex has been licensed in the UK to treat MS - it contains THC and CBD.

Doctors could, in theory, prescribe it for other things outside of this licence, but at their own risk.

MS patients prescribed Sativex, who resupply it to other people, also face prosecution.

Another licensed treatment is Nabilone - it contains an artificial version of THC and can be given to cancer patients to help relieve nausea during chemotherapy.

Source: NHS Choices

Case study: 'Amazing impact on my son's life'

Belfast woman Jennifer Gillen's son Henley, 11, suffers from epilepsy - has done since he was eight.

She said she started giving him CBD oil after a reaction he took to pharmaceutical drugs which last year "nearly killed him".

"We didn't take this decision lightly to start him on this - we have found it has had an amazing impact on Henley's life.

"The seizures are being kept at bay - he can have up to 30 seizures a day but by using the oil he's maybe only having four or five."