Consultants will be able to prescribe the drug after a campaign by mother Charlotte Caldwell prompted a change in the law.
Patients in the UK can be prescribed medicinal cannabis from today, if it is agreed by a specialist doctor.
For the first time in this country, consultants have been given the option to prescribe products containing cannabis, cannabis resin or cannabinol, if they feel their patients could benefit from it.
It comes after a summer of campaigning by parents including Charlotte Caldwell, whose son Billy has severe epilepsy.
She went to Canada to procure the cannabis oil she says controls his seizures, but was not allowed to bring it back into the UK.
Her fight to keep the drug led to a policy review by home secretary Sajid Javid who brought in today's law change after advice from experts on the Advisory Council on the Misuse of Drugs and the UK's Chief Medical Adviser.
"For me what started off as a journey which was about the needs of my little boy actually turned into something, proved to be something, a lot bigger. It proved to be the needs of a nation," Charlotte told Sky News.
"Medicinal cannabis gave me back my right as a mummy to hope, but the most important thing medicinal cannabis has done is given Billy back his right to life."
Other patients who have had to get their cannabis illegally up until now agree.
Faye Jones, from Reading, has rheumatoid arthritis and was put on strong chemotherapy drugs to control the pain.
Discovering cannabis, she said, was like "waving a magic wand" - and after being sofa-bound, she can now can get up, go to the gym, and hold down a successful career as an executive assistant.
But, she says, getting hold of the drug on the black market is "expensive and time consuming. Obviously I have to break the law to acquire my medicine.
"I used to bemoan the amount of time I spent sitting in pharmacies waiting to pick up prescriptions when I was first diagnosed - I would absolutely love to go to a pharmacy and pick up a prescription again now."
Carly Barton agrees. A fibromyalgia sufferer from Brighton, she found cannabis ended six years of intense and constant nerve pain.
"I've still been in some states, in agony in the dark, meeting people in parks," she tells Sky News.
"Going to meet people I've never met before in abandoned industrial car parks. And in the middle of night in the dead of winter and I'm on a walking stick and I've got nothing to protect myself.
"I could handle myself, I could get out of a situation like that. But there are a lot of people a lot more vulnerable than I am, or was, being put in that situation.
"Hopefully some of those patients will be able to go to their doctors or go to the chemists where medicine belongs, and go to a place that's safer for them to access."
The new NHS England guidelines say doctors should only prescribe cannabis-based medicine if other options have been exhausted, for conditions including rare childhood epilepsy and multiple sclerosis as well as to help deal with nausea from chemotherapy drugs.
Only hospital specialists on the General Medical Council register will be able to write such prescriptions.
"I think we're being quite cautious in saying it can only be prescribed by consultants and that's probably a good idea in the first instance," Dr. Saoirse O'Sullivan, an associate professor at the University of Nottingham, says.
"But there's a lot of people who aren't under consultant care that won't now have access. Two of the main reasons people use cannabis is for anxiety and depression, and they're not likely to be under consultant care.
"They're more likely just to be seeing their GP, and GPs won't be able to prescribe cannabis based products. So I think there's going to be a lot of disappointed people."
Clark French of the United Patients Alliance, which also campaigns for access to cannabis based medicine, thinks more could be done.
"The law change is brilliant and we're really pleased to see movement - but it's by no means enough," he told Sky News.
"A lot of patients are going to want to access cannabis, they're going to be sent to their specialist and the specialists are going to be inundated with requests. Really what the government needs to do is allows GPs to prescribe cannabis as well."
Some are concerned that allowing medicinal cannabis could normalise the drug for recreational use.
The home secretary says this law change is to help "patients with an exceptional clinical need, but is in no way a first step to the legalisation of cannabis for recreational use."
For Charlotte Caldwell, the issues are entirely separate.
"Never once has it crossed my mind or any of the families that I've spoken to, we've never had thoughts about recreational use, and it's simply because it's all about the needs of - in Billy's case, his medical needs.
"It's all about the need to get my little boy well."
Here is the link to the new NHS Englands Guidance on Cannabis based Products for Medical Use: