Cannabis: Promise, risk and controversy
Cannabis is bad for you, cannabis is good for you - confused?
That's not surprising. Complicated and controversial, cannabis is revealed by recent science to have a dual personality, with a dark side and a more positive one. Radio 4's PM programme is this week running a whole series on cannabis, and the debate surrounding it.
Key to understanding this strange plant are two of the ingredients that make it up, known by their initials as THC and CBD.
I asked Prof Val Curran of University College London to describe how they work and she came up with a memorable answer:
"In a way, THC and CBD are a bit like yin and yang. The THC makes you stoned, but it can also make you anxious. It can also make you feel a bit psychotic, and it will seriously impair your memory.
"The other side of the yin/yang is CBD, which has almost the opposite effects. CBD calms you down, it has anti-psychotic properties and it also offsets the effects on memory, so that on CBD-containing cannabis you're less likely to forget what's going on."
So the first step to understanding cannabis is to realise how it can vary, how different types contain very different quantities of these polar opposites, with dramatically different outcomes.
The weed so familiar to many of my generation was characterised by a relatively balanced amount of THC and CBD.
Today, the vast majority of cannabis on sale on the streets is unrecognisably stronger.
Known as skunk, it contains a far higher proportion of THC - as much as 15% - which produces a much more powerful high, making it more appealing for users.
But, at the same time, because it hardly contains any of the CBD that might lessen its effects, the risks are correspondingly greater.
Prof Curran is among those worried about its potency.
"What concerns me is that on this high-THC skunk, people will experience more memory problems, which could affect how well they do at school. And in terms of addiction, 10% of people who use it will become addicted to the drug."
According to a study by two researchers at UCL, Dr Tom Freeman and Dr Adam Winstock, the strongest cannabis increases the risk of addiction, along with memory loss and paranoia.
And in a trial to explore ways of helping addicts, they are giving drug users medication based on cannabis itself. The hope is that administering doses of CBD, the more benign ingredient of cannabis, might make it easier for habitual users to wean themselves off the lure of the more potent element, THC.
Dr Freeman told the BBC: "We think that CBD can reverse long-term changes which happen when you smoke cannabis repeatedly, and in people who smoke a lot of cannabis it'll help them quit.
"It blocks the effects of THC and it reduces anxiety and paranoia. If this trial is successful, then we will have found the first effective drug treatment for cannabis dependence."
Meanwhile, new evidence has surfaced that will stir the long-running debate over whether - or to what extent - cannabis can trigger psychosis.
New research published this week in the Lancet Psychiatry suggests a connection, a finding which is most relevant to people already vulnerable to mental illness.
The study, conducted in south London, involved some 800 people - about half of them users, the rest not.
One of the authors, Prof Sir Robin Murray of King's College London, says it's clear that regular use of highly potent skunk has a real impact.
"We found that smoking cannabis, particularly of the high-potency forms, was associated with an increased risk.
"If you smoke high-potency skunk at all, then you are three times more likely to be psychotic. If you smoke high-potency cannabis every day, you are five times more likely to be psychotic."
And at this point we come back to that yin and yang of cannabis. While this new research finds that the strongest cannabis, laden with THC, can be linked to psychosis, it turns out that the gentler twin, CBD, might possibly be useful in treating it.
Prof Murray, though cautious, highlights recent studies.
"If you give THC to normal volunteers, you can make them psychotic, but if you pre-treat them with CBD, you can prevent that happening.
"So this made us think - would it be possible to actually treat psychosis with CBD? So there's one encouraging study, which suggests that CBD is useful in the treatment of psychosis, but it's still very early days yet."
So running in parallel with concerns about cannabis is another world of optimism about its uses.
In Colorado, there is much excitement about a medication called Charlotte's Web, derived from cannabis and named after a girl who took it as a treatment for her epilepsy.
Such is the potential of what's seen as a wonder drug that the Mattison family sold up their business in Tennessee and moved to Colorado purely so that their daughter Millie, who's two years old and epileptic, could receive Charlotte's Web.
Her seizures, soon after birth, were so severe that she had been given very little chance of surviving.
But her mother Nicole told me that the drug proved immediately beneficial, transforming Millie's life almost at a stroke.
"It's miraculous. The first time we gave her oil, within 15 minutes her eyes were open, and I almost felt like I was in a movie.
"It was crazy, you wouldn't believe it unless you saw it."
Here in the UK, the only legal medicine derived from cannabis is for sufferers of multiple sclerosis (MS), a product called Sativex made by GW Pharma.
But now the company, the only one with a licence to grow cannabis in the UK, has developed another formulation which is being tested to treat epileptic conditions like Millie's.
The trial, with 80 patients, is now in its second stage and is being run by the University of Edinburgh.
The scientist in charge of the process, Dr Richard Chin, says that so far the results look promising, not just to control seizures but - remarkably - to prevent them as well.
"One of the interesting things about cannabidiol (CBD) is that it shows not just anti-seizure effects, but it also curiously seems to have an effect on cognitive and behavioural problems, which are very highly represented in people with epilepsy.
"So it doesn't seem, on preliminary data, as if it's just an anti-seizure medication. It may actually be an anti-epilepsy medication in its wider sense, and what I would hope is that it may open up a completely new avenue of treatment options for patients with epilepsy."
For thousands of years cannabis was used medically. But only now is research revealing why that's possible and how it can be put to best use.
These are relatively early days but, on the horizon, researchers see potential for the CBD in cannabis to help with everything from easing the pain of cancer to tackling autism.
At the same time, science is also unpicking the full implications of the potent stuff being dealt on our streets.
You can listen to my full report tonight on the PM programme on BBC Radio 4 as part of Cannabis Week.
Follow me on Twitter: @davidshukmanbbc