The 'horse tranquilliser' Ketamine has been used as a prototype for a new drug which could be approved to treat depression in the form of a nasal spray by the end of the year, experts say.

Esketamine, the 'fast acting' treatment for depression, could be approved for use in the UK in November — and is already sold in the US under the brand name Spravato.

It is hoped the drug will revolutionise the treatment of chronic depression by easing symptoms within hours, The Guardian reported.

But critics have slammed the treatment, labelling it an 'over-hyped rip-off'.

Esketamine, the 'fast acting' treatment for depression, could be approved for use in the UK in November — and is already sold in the US under the brand name Spravato

Patients who take the drug will have to spend two hours under supervision each week to ensure they don't develop any negative side effects.

They include hallucinations, dizziness and anxiety.

And the amount of supervision needed is likely to become a barrier to the drug becoming a common treatment option.

Professor Alan Young, director of the Centre for Affective Disorders at King's College London, told The Guardian: 'It's got a different pharmacology. It's not just the same old steam engine; it seems to work more quickly.'

While other drug treatments work by increasing the level of serotonin in the brain, ketamine affects the brain chemical glutamate, the substance nerves use to connect with each other.

Serotonin creates feelings of well-being and happiness. While increased nerve connectivity could increase brain function.

Anti-depressant drugs usually take up to eight weeks to have any effect. But the nasal spray — which contains a lower dosage than what drug-users take illegally — can treat depression within hours.

The European Medicines Agency and the UK health regulator will make a decision in November of licensing the drug, which Johnson & Johnson already sells in the US.

If approved it will become available in private clinics and a decision on whether to roll it out to the NHS will be made next March.

While other drug treatments work by increasing the level of serotonin in the brain, katamine affects the brain chemical glutamate, the substance nerves use to connect with each other (file image)

But while Young believes the spray could vastly improve the lives of millions of people who have not responded to conventional treatment, critics say it could act as a gateway to more serious drug-taking.

And Wes Boyd, associate professor of psychiatry at Harvard, said some clinical trials had not shown a large enough difference between those who were given the drug, and those who were given a placebo.

He added: 'The cost of esketamine is dramatically high and comes with a very large and scary side-effect profile.

It is so potentially dangerous that clinicians are required to sit with patients for two hours after they are administered the drug.

'The upshot is that the drug is an over-hyped rip-off.'

Trials have been mostly short-term, there is little understanding of the consequences of long-term use and the drug costs £25,800 per person per year in the US.

Ketamine, currently used by doctors and vets as a painkiller and tranquilliser, acts on the brain chemical glutamate, and in animal studies has been shown to restore brain connections which have shrunk after long periods of depression.