People with severe epilepsy will be able to access a cannabis-based medicine on the NHS from early next year after it was fast-tracked for use.
NHS England said doctors would be able to prescribe Epidyolex from 6 January.
It will be for children from age two, as well as adults, but some campaigners warn it is "too little too late".
Clinical trials have shown the oral solution, which contains cannabidiol (CBD), could reduce the number of seizures by up to 40% in some children.
The medicine will be used to treat two rare, but severe, forms of childhood epilepsy - Lennox Gastaut syndrome and Dravet syndrome - which can cause multiple seizures a day.
Epilepsy Action's chief executive Philip Lee welcomed the announcement, saying it "brings much-needed hope and could be life-changing for some".
However, he added that Epidyolex was not "a silver bullet" and there was more work to be done to "collect robust high-quality evidence of the effectiveness of other cannabis-based medicines".
Medical cannabis campaigner Peter Carroll said it was "too little, too late" as he urged action towards making medicinal cannabis with CBD and tetrahydrocannabinol (THC) available for families in need.
THC is the psycho-active component of cannabis.
Speaking to BBC News, he said: "What's shown to have a transforming effect for children in desperate need is a CBD medicine with a little bit of THC, but those are unlicensed in the UK at the moment."
Mr Carroll added: "The law was changed in November 2018 so that specialist doctors could write a prescription for medical cannabis with the CBD and THC, even though they are unlicensed.
"Shockingly, to our knowledge, not a single prescription for the medicine with those two parts was issued on the NHS since the law was changed."
An official NHS review earlier this year found that a "lack of evidence" was holding back cannabis medicines, which specialist doctors have been allowed to prescribe in the UK since the law change last year.
Decisions on drug availability are devolved around the UK.
It is estimated there are 3,000 people with Dravet and 5,000 with Lennox Gastaut syndrome in England.
NHS chief executive Simon Stevens said that thousands of people would now have access to the treatment "which has the potential to make a real difference".