The introduction of medicinal cannabis has “opened a Pandora’s box”, with patients believing the drug can cure multiple conditions, the Chief Medical Officer has said.

Dame Sally Davies told MPs that despite being recently legalised for medical use, there is currently insufficient evidence to prove the products are both effective and safe.

She also expressed “concerns” about the safety of the drugs for some patients, warning that until widespread trials had been completed medicinal cannabis should only be prescribed as a “last resort”.

Her comments follow a change in the law last November which removed the products from Schedule 1 of the Misuse of Drugs Regulations, which had banned their use in all circumstances.

The move came after the high-profile campaign by supporters of Alfie Dingley, a young boy who had been forced to travel to The Netherlands to receive cannabis oil prescriptions to treat his severe epilepsy.

The strict ban had also prevented any clinical trials taking place, without which the medicinal cannabis products cannot formally be licensed.

It means doctors who prescribe the products enjoy less legal protection if something goes wrong.

Last week The Telegraph reported a widespread refusal to prescribe the drugs by NHS doctors, despite the change to the law.