My son, J, has autism. He's also had two serious operations for a spinal cord tumour and has an inflammatory bowel condition, all of which may be causing him pain, if he could tell us. He can say words, but many of them – "duck in the water, duck in the water", for instance – don't convey what he means. For a time, anti-inflammatory medication seemed to control his pain. But in the last year, it stopped working. He began to bite and to smack the glasses off my face. If you were in that much pain, you'd probably want to hit someone, too.

J's school called my husband and me in for a meeting about J's tantrums, which were affecting his ability to learn. The teachers were wearing Tae Kwon Do arm pads to protect themselves against his biting. Their solution was to hand us a list of child psychiatrists. As autistic children can't exactly do talk therapy, this meant using sedating, antipsychotic drugs like Risperdal.

Last year, Risperdal was prescribed for more than 389,000 children in the US – 240,000 of them under the age of 12 – for bipolar disorder, ADHD, autism and other disorders. Yet the drug has never been tested for long-term safety in children and carries a severe warning of side-effects. From 2000 to 2004, Risperdal, or one of five other popular drugs also classified as "atypical antipsychotics", was the "primary suspect" in 45 paediatric deaths, according to a review of US Food and Drug Administration (FDA) data by USA Today. When I canvassed parents of autistic children who take Risperdal, I didn't hear a single story of an improvement that seemed worth the risks. A 2002 study on the use of Risperdal for autism, in The New England Journal of Medicine, showed moderate improvements in "autistic irritation" – but the study followed only 49 children over eight weeks, which limits the inferences that can be drawn from it.

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We met with J's doctor, who'd read the studies and agreed: No Risperdal or its kin. The school called us in again. What were we going to do, they asked. As an occasional health writer and blogger, I was intrigued when a homeopath suggested medical marijuana. Cannabis has long-documented effects as an analgesic and an anxiety modulator. Best of all, it is safe. The homeopath referred me to a publication by the Autism Research Institute describing cases of reduced aggression, with no permanent side- effects. Rats given 40 times the psychoactive level merely fall sleep. Dr Lester Grinspoon, an emeritus professor of psychiatry at Harvard Medical School who has been researching cannabis for 40 years, says he has yet to encounter a case of marijuana causing a death, even from lung cancer.

A prescription drug called Marinol, which contains a synthetic cannabinoid, seemed mainstream enough to bring up with J's doctor. I cannot say that with a few little pills everything turned around. But after about a week of fiddling with the dosage, J began garnering a few glowing school reports: "J was a pleasure have in speech class," instead of "J had 300 aggressions today."

But J tends to build tolerance to synthetics, and in a few months we could see the aggressive behaviour coming back. One night, I went to the meeting of a medical marijuana patient advocacy group on the campus of the college where I teach. The patients told me that Marinol couldn't compare to marijuana, the plant, which has at least 60 cannabinoids to Marinol's one.

***

Rhode Island, where we live, is one of 13 states where the use of medical marijuana is legal. But I was resistant. My late father was an anaesthesiologist, and compared with the precise drugs he worked with, I know he would think marijuana to be ridiculously imprecise and unscientific. I looked at my son's tie-dye socks (his avowed favourite). At his school, I was already the weirdo mom who packed lunches with organic kale and kimchi and wouldn't let him eat any "fun" foods with artificial dyes. Now, I'd be the mom who shunned the standard operating procedure and gave her kid pot instead.

I thought back to when J was 18 months old. We were vacationing on the Cape, and, although he just had the slightest hitch in his gait, I was sure there was something wrong. His paediatrician laughed. I called back repeatedly until a different doctor agreed to see us. J was taken in for emergency surgery, to remove a tumour that was on the verge of inflicting irreparable damage. Sometimes, you just have to go with your gut.

And yet, I still hesitated. The Marinol had been disorienting enough – no protocol to follow, just trying varying numbers of pills and hoping for the best. Now we were dealing with an illegal drug, one for which few evidence-based scientific studies existed, precisely because it is an illegal drug. But when I sent J's doctor the physician's form that is mandatory for medical marijuana licensing, it came back signed. We underwent a background check with the Rhode Island Bureau of Criminal Identification, and J became the state's youngest licensee.

Having a licence, however, is different from having access to marijuana. While California has a network of "compassion centres," basically pharmacy-like storefronts that provide quality product from registered growers, Rhode Island's Republican governor has consistently vetoed that idea, despite the local stories of frail patients being mugged in downtown Providence as they go in search of pot. We weren't about to purchase street marijuana, which could be contaminated with other drugs, so we looked into growing the pot ourselves. But by law, medical marijuana must be grown indoors, and it requires a separate room with a complex system of hydroponics, fans and precise lighting schedules. (This made me wonder how much THC, the main psychoactive substance found in cannabis, was actually in the spindly plants the high school goofballs I knew grew in their closets).

The coordinator of our patient group introduced us to a licensed grower. A recent horticulture school graduate, he'd figured out how to cultivate marijuana using a custom organic soil mix. His e-mail signature even quoted Rudolf Steiner. The grower arrived at our house with a knapsack containing jars of herbs. We opened the jars to sniff the different strains of "bud" – Blueberry, which did smell fleetingly of wild blueberries, and Sour Diesel, which had a rich, winey scent. The grower had also cured some leaves for tea, and he brought a glycerine tincture, a marijuana distillate in olive oil (yes, organic), cookies (ditto), and a strange machine that looked, fittingly, like a lava lamp. Basically an almost-bong, this vaporiser heated the cannabis without producing carcinogenic smoke.

For most adults, the vaporiser is the delivery method of choice, as it allows the patient to feel the effects immediately and adjust the dose precisely. J gamely put his mouth on the valve and let us squeeze a little smoke into him. It shot right back out of his nose. He looked like Puff the Magic Dragon. The grower left us with a month's worth of marijuana tea, glycerine, and olive oil – and a cookie recipe. No buds. We paid $80 (£50).

We made the cookies with the marijuana olive oil, starting J off with half a small cookie, eaten after dinner. J normally goes to bed around 7.30pm; by 6.30 he declared he was tired and conked out. We checked on him hourly. As we anxiously peeked in, half-expecting some red-eyed ogre from Reefer Madness to come leaping out at us, we saw instead that he was sleeping peacefully. Usually, his sleep is shallow and restless. J also woke up happy.

But in a few days, J decided he didn't like the cookie anymore and smashed it with his fist. We brewed him the tea, which smelled funky and grassy. He slurped it down, but it didn't seem to do much. Many of the psychoactive compounds in marijuana are fat soluble, so I added a dropperful of the oil that we used in the cookies. That made him sleepy-looking but still aggressive. It became clear that when J ingested pot orally, it took two hours to see the results, and by then there wasn't much we could do to dial the dose up or down. The grower visited us again to give J another try at the bong, but with little success.

Perhaps J needed a little time to get off the Marinol. After two weeks, we noticed a slight but consistent lessening of aggression. And he wasn't nervously chewing holes in his shirts.

***

A month or so into the treatment, it was still too early to know if we could find a dose and mode of delivery that would give us consistent results. Even if J could learn to use the vaporiser, it costs $600 and would leave the house reeking of pot. And we didn't want to get too dependent, because of the inherent limitations. Though we'd love to calm J with pot so that he can visit his grandmother in Minnesota, bringing a controlled substance on the plane isn't the best idea.

But since we started him on his "special tea," J's little face, which is sometimes a mask of pain, has softened. He's smiled more. For most of the last year, his individual education plan at his special-needs school was full of blanks, recording "no progress" because he spent his whole day an irritated, frustrated mess. But soon after starting on the tea, his reports began to show real progress, including "two community outings with the absence of aggressions".

My husband and I are both academics and writers (me, novelist and essayist; he, historian), given to close observation and note taking. It was these habits that finally helped us see our son's allergic sensitivity to certain foods and seek advice from a gastroenterologist for his behaviours – aggression and chronic diarrhoea – instead of the recommended psychiatrist. (Gut pain and digestive problems, coined as "autistic entercolitis", are now considered a common biological affliction of many autistic children).

At first we weren't sure if we were seeing results from the cannabis, but after about three months, which included weekly consultations with our grower as we experimented with different strains, we observed a much happier and outgoing child – who did not act or appear "stoned" in any way. Four months in, J came home from school and I noticed something different. Pre-pot, J ate the collars of his shirts, teasing his clothes apart and swallowing the threads. There's a name for this disorder – pica (pregnant women sometimes chew on chalk). It got so bad he ate his pyjamas and we had to start dressing him in organic cotton shirts. Then one day he came home from school wearing a whole shirt.

J's school reports improved too. At one parent meeting, his teacher produced the latest "aggression" chart, showing attempts or instances of hitting, kicking biting or pinching other people. For a year he had scored an average of 30 to 50 aggressions a day, with a high of 300. The latest data showed days, sometimes consecutive, with zero aggressions. And on the school bus, J has transformed from a child who has hit the driver in the face and bitten people into a sparkly eyed boy who says hi and quietly takes his seat.

***

There's a twist to this happy story, though. The aggression has eased but J's autism has become more distinct. His vocal outbursts – screams, barks, yips of happiness – still happen and while our home is no longer full of thrown food, broken dishes and scratched faces, we still see people in the local area react to a family that remains different – and not always to their liking. There's a father on the next street who stops playing ball with his son when we approach. A mother won't make eye contact and ignored a party invitation. Most people responded well to J but sometimes we feel we're being shunned.

Marijuana isn't a miracle cure for autism. But in our son's case it eases his pain and inflammation so dramatically that he can participate in life and learning again. It also protects him from the sometimes dangerous side-effects of pharmaceutical drugs. We have settled on a good strain (White Russian, a favourite pain-reliever for end-stage cancer patients) and a good dose. And now he's not in pain, J can go to school instead of a children's psychiatric hospital, where all too many of his peers end up as a result of violent behaviour.

When I think of the embarrassment I may feel if my colleagues see this article, or teachers or parents at J's school, or his less open-minded doctors, I pause. Although I occasionally smoked pot as a teenager (believe me, in northern Minnesota, there was not much else to do), now that I'm a law-abiding adult, all the scary anti-drug messages are flashing in my brain. But when I researched cannabis the way I did conventional drugs, it seemed clear that marijuana wouldn't harm J, and might help. It's strange that the virtues of such a useful and harmless botanical have been so clouded by stigma. Even the limited studies that have been done suggest marijuana's potential as an adjunctive therapy for cancer. Marijuana, you need some re-branding. Maybe a cool new name.

One of the biggest tests for J through this journey was a visit from Grandma. The last time she came, over Christmas, J hit her during a tantrum. This time, we gave him his tea, mixing it with goji berries to mask any odour, although it occurs to me that my mother, a Korean immigrant, probably doesn't even know what pot smells like (it actually smells a lot like ssuk, a Korean medicinal herb). She remarked that J seemed calmer. As we were preparing for a trip to the park, J disappeared, and we wondered if he was going to throw one of his tantrums. Instead, he returned with Grandma's shoes, laying them in front of her, even carefully adjusting them so that they were parallel and easy to step into. He looked into her face, and smiled.

What are the downsides to this experiment?

By Jeremy Laurance, Health Editor

The first reaction of most parents to Marie Myung-Ok Lee's story is likely to be one of surprise, shock, even horror. What is she doing turning her nine-year-old son into a pot-head? Has she not heard of the dangers of cannabis smoking to the mental health of adolescents, never mind the disorienting effects of an intoxicating substance on one so young?

Possibly this will be their second and third reactions, too. Ms Myung-Ok Lee was giving her son, J, cannabis to relieve pain (from his spinal tumour and inflamed gut), not just to treat his autism. Even so, the stigma that surrounds illegal drugs is so deeply entrenched, just because they are illegal, that many people are simply not prepared to weigh up their benefits and harms.

We have seen in the row this week over the sacking of the UK Government's chief drugs adviser, Professor David Nutt, how the debate over drugs is driven more by fear, emotion and political calculation than by scientific evidence. The Labour Government, facing possible annihilation at the next election, is anxious to be seen to be tough on drugs – so the outspoken Professor Nutt had to go.

As an academic, Ms Myung-Ok Lee is perhaps better placed than many to resist the voices of unreason and take a cool look at the evidence. Cannabis, as she points out, is already prescribed as a pain killer, as an anti-nausea agent for cancer sufferers and as a treatment for multiple sclerosis. In all these areas it has been shown to be effective, though there is debate about just how effective. In the UK, it is available as Sativex, a spray taken under the tongue, which contains a cannabis extract. More than 1,200 patients in the UK have received it for relief of symptoms associated with multiple sclerosis. It is not, however, prescribed to nine-year-olds (or anyone under 18).

Ms Myung-Ok Lee started her son on medicinal cannabis, and then went a step further by giving him the herbal kind, as a tincture or baked in a cookie. This, too, is not without precedent – among adults. There have been frequent reports of patients smoking cannabis and gaining relief from pain or the spasticity associated with multiple sclerosis, and in the UK when they have been prosecuted for possession of a controlled drug, the courts have shown leniency.

But in trying herbal cannabis on her son, Ms Myung-Ok Lee and her doctor have stepped beyond even the anecdotal evidence, into the unknown. J became Rhode Island's youngest ever patient licensed to use marijuana for medical reasons.

She acknowledges it is an experiment, but she reasons that as cannabis has low toxicity and is safer than most other drugs, the risks are low. Any parent, confronted with a screaming, suffering child who is so distressed that he smashes things, hits people and tears at his clothing with his teeth, must feel sympathy for her. In that situation, which of us would not try anything to ease our child's pain? Moreover, the experiment appears to have worked – at least for the first few months.

The difficult questions are: will the effect last? Will there be a downside to using the drug in one so young? Is the effect real? The last question is the trickiest. Children grow and change and those with autism are no different from the rest. The changes his parents have noticed in J might have happened anyway, as part of his natural development. The cannabis could turn out to be a coincidental factor, with zero impact on his condition. It was coincidence that led to the scare over MMR and autism – because the first symptoms of the condition typically occur around 14 months which is the age at which babies receive their first MMR jab.

It would be a disaster if cannabis came to be seen as a panacea for children in the same situation, on the basis of this anecdotal report. As always in science, we need more evidence.