The first medical marijuana dispensaries in Pennsylvania have opened their doors over the past few days. Thousands of patients with serious medical conditions have registered to access the drug, which comes in oils, patches and vaporisers, rather than smokable leaf form. The historic first purchase in the state was by Diana Briggs, the mother of a teenage son with severe epilepsy. “I am beyond thrilled,” she said. “There’s no more fear, no more stress for our family.”

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Briggs campaigned for this moment after seeing how the drug reduced her son’s seizures from more than 400 a day to fewer than 50. As I know from my own daughter, who also has serious epilepsy, this is a miserable condition when poorly controlled by conventional medicine. Even after 24 years, I still find seizures distressing to watch – seeing the fear on her face when they start, holding her thrashing body tight for several minutes, then making her comfortable when she is left drained for hours. There is nagging fear in the knowledge that these episodes can kill.

Contrast the tears of joy for that mother in the US with the concern facing another mother in Britain. Hannah Deacon sees her six-year-old son, Alfie Dingley, endure up to 30 violent seizures a day. His type of epilepsy is so aggressive that he ended up in hospital 48 times in one year. Then he went to the Netherlands, and enjoyed 24 days without a single attack. The difference seems to have been made by a cannabis-based medication – just three small drops of oil – prescribed by a paediatric neurologist. “We’ve found something that makes him happy,” said his mother.

But since returning to Britain, Alfie has had to return to injections of steroids that are less effective and might eventually cause his organs to fail. This child is another innocent victim of the global war on drugs – perhaps the most foolish aspect of this futile 50-year fight has been the ban on medical marijuana. Now, Pennsylvania has become the 29th state in America to permit its use, while Italy and Germany are among European nations considering this shift.

Families with epilepsy are allowed to routinely use stronger, more addictive drugs such as barbiturates

Slowly, however, things are changing. But there seems still to be befuddled stupor in Whitehall. This is perhaps unsurprising, under a prime minister whose policy record on drugs during her time as home secretary showed clear disdain for evidence. Yet it is callous and morally confused to stop patients accessing a cheap, safe and readily available drug that could improve life so dramatically. It is also irresponsible to ignore its fiscal potential, given the importance of the pharmaceutical sector to our economy. Among those planning to dispense the drug in Pennsylvania is a firm run by a former British policeman, Michael Abbott. His company, Columbia Care, has won licences in 12 states, and its boss would love nothing more than to be able to offer the drug to his home country. At one of his New York dispensaries I met his chief pharmacist. She explained she was at first embarrassed to be linked to the cannabis trade, and became a convinced advocate only after seeing changes in patients using the drug, especially for chronic pain, late-stage cancer and complex child epilepsy.

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This does not have to be a Trojan horse for wider drug reform. And yes, much of the evidence is anecdotal since cannabis research has been hampered by illegality – although it is worth noting that Big Pharma seems resistant to an emerging industry that has reduced opioid deaths in American states where it is permitted to operate. Besides, why are families with epilepsy allowed to routinely use stronger, more addictive drugs such as barbiturates and benzodiazepines in their homes but not cannabis?

Polls find that more than two-thirds of Britons and more than half of MPs back the use of medical marijuana. Perhaps we will see a revolt on Friday, when the Labour MP Paul Flynn tries to reschedule cannabis for medical purposes in a private bill. For where is the compassion or sense in letting children such as Alfie needlessly suffer?

• Ian Birrell is a former deputy editor of the Independent and worked as a speechwriter for David Cameron during the 2010 election campaign