Cannabis-based medicinal products will be available in Northern Ireland and the rest of the UK from today.
They can only be obtained if they have been prescribed by a specialist hospital doctor for patients with "an unmet clinical need".
The move follows a high profile campaign for the relaxation of regulations around cannabis-based products for medicinal use.
Billy Caldwell, 13, of Castlederg, Co Tyrone, hit the headlines during the campaign.
Earlier this year, he received national attention when his medicinal cannabis was confiscated by the UK Home Office after a trip to Canada to access the medication.
Billy suffers from a severe form of epilepsy and his mother Charlotte says medicinal cannabis products help to ease his symptoms. She spent years campaigning for the law change.
The British Home Office then gave him a short-term licence and the UK Department of Health issued an emergency licence to allow him access to cannabis oil medication.
Belfast Health Trust secured a licence to administer the medication to Billy at the Royal Victoria Hospital.
He was then given permission to receive the medication at his Castlederg home.
In early October, UK Home Secretary Sajid Javid announced a decision to reschedule the products, relaxing the rules around the circumstances in which they can be given to patients, after considering expert advice from a specially commissioned review.
Ms Caldwell said she wept happy tears at the move.
"What started out as a journey for me as Billy's mummy to help my little boy has become something much bigger," she said.
From today, cannabis-based products for medicinal use can be prescribed by specialist hospital doctors for their patients with an unmet clinical need.
In a statement, the UK Department of Health said: "It is important that prescribing decisions are taken by expert clinicians informed by evidence on quality, safety and effectiveness.
"Cannabis-based products for medicinal use will not be available from General Practitioners.
"It is important to note that the rescheduling of cannabis-based products for medicinal use does not pave the way towards legalising cannabis for recreational use.
"The penalties for unauthorised supply and possession will remain unchanged."
A spokeswoman for Minister for Health Simon Harris said Ireland currently has a licencing system for the use of medicinal cannabis.
"This allows any consultant to apply to the Minister for a licence to prescribe cannabis where the consultant believes it can benefit the patient."
The minister is seeking to overhaul that system and modernise it further, she said.
Mr Harris asked the HPRA to undertake a policy review in 2017 and on their advice has been developing an access programme for patients with certain conditions where other treatments have failed.
"Department of Health officials are working on secondary legislation which will underpin the Access Programme and reschedule specified medical cannabis products.
"This work will be finalised once appropriate cannabis-based products, to be included in the legislation, can be sourced
"That is proving to be a difficulty and Department of Health officials are working with their European counterparts to establish how this obstacle can be overcome."