The duration of the effect is still a problem.

 

Some relapse within days, while others have found they benefit for around three months and have since had additional doses of ketamine.

 

There are also some serious side-effects including one case of the supply of blood to the brain being interrupted.

 

Doctors say people should not try to self-medicate because of the serious risk to health outside of a hospital setting.

 

"It is exciting, but it's not about to be a routine treatment as where we need to be going is maintaining the response... it's not about to replace prozac."

 

However, it does offer a new avenue of research into a field that has struggled to find new treatments for depression.

 

'Something chemical'

David Taylor, professor of psychopharmacology at King's College London, told the BBC: "In these kinds of patients, spontaneous remission almost never happens, people going to these clinics are at the end of the road.

 

"It shows that depression is something chemical, that it can be reversed with chemicals, it dispenses for once and for all that you can just pull your socks up.

 

"What restricts it is the potential for disturbing psychological adverse effects and the route by which is given - intravenous - does restrict it to a small number of people."

 

He said in the future drug companies would develop a chemical that had the benefits, but without the side-effects, and that could be taken by something such as an inhaler.

 

The Home Office is reclassifying ketamine in the UK to be a class B drug, although it is already used in medicine for the treatment of back pain and as an anaesthetic.