Doctors will be able to prescribe cannabis products to patients in the UK from today, but there are concerns that only a limited number of people qualifying for treatment.
The medicines can only be prescribed by a specialist doctor - not a GP - on a case-by-case basis.
New NHS guidance say a decision to prescribe cannabis products should only be made where other treatment options have been exhausted - meaning some, including Deryn, would not be eligible for treatment under the new guidelines despite potential benefits to health.
Deryn's mother, Callie Blackwell, told ITV News that "very few are going to benefit from this."
"Once the drugs are made they will be far too expensive. So unfortunately, there are many people - care givers and patients alike - who are still going to be criminals just for keeping themselves healthy and those they care about alive," she said.
Who is eligible?
But, some have reservations about the rolling out of the drug.
Consultant Rajesh Munglani believes that doctors may become inundated with patients wanting new medicines, putting them in a difficult position.
"There is no evidence unfortunately that cannabis actually gives a good quality of life in most of our patients," Dr Munglani.
"Of course there are one or two people who say it has been really dramatic, but every time we write a prescription we know it is four times more likely that it will harm somebody than help somebody."
Previously medicinal cannabis could only be accessed in very rare cases with a special licence from the Home Office.
In July, Sophia Gibson, a seven-year-old girl from Newtownards in Northern Ireland was for medicinal cannabis to treat her Dravet syndrome, a severe form of epilepsy which causes what her family describe as “frequent and dangerous fits”.
GW Pharmaceuticals is the leading producer of cannabis-based medicines - and the only UK company with a licensed and regulated product being used in Britain and across the world.
Its chief operating officer, Chris Tovey, is convinced that legalising the drug for medical use will enhance research.
"I think the future is really exciting. As well as reducing the stigma, I think what it is going to encourage is a better understanding of cannabis-based medicines," Mr Tovey said.
"The reforms and changes are going to encourage more research, and it's also going to help the medical community - understanding much better what is the potential."
NHS England has issued guidance on prescribing medicinal cannabis.
But the government insists that doctors are in the best position to decide whether or not to prescribe the drug.
Health Secretary Matt Hancock said: "Doctors need to use their clinical judgement and having guidance in place helps.
"Ultimately the need to treat an individual person and the responsibility for that falls on the shoulders of a doctor - that's what they do."
While medicinal cannabis can now be prescribed, the Department of Health is keen to stress that this is not the first step towards legalising the drug for recreational use.