Cannabis has been around for an awfully long time. There is evidence that it was being used in China as a medicine as long ago as 2600 BC, and I’m fairly sure that at least some Chinese people would have been using it recreationally.

But what has happened over the last few years is unprecedented, with an enormous surge in interest in cannabis-related products, to the point where it is being added to everything from dog food to chocolate to shampoo. The problem with all this hype is there is remarkably little evidence about genuine effectiveness.

It is certainly an interesting plant, containing more than 500 different compounds, many of which interact with ‘cannabinoid’ receptors in our bodies. One reason why cannabis has such wide and varying effects is because our bodies are riddled with these receptors, which help regulate sleep, stress, pain, appetite, memory and mood.

This in turn helps explain why people are now using cannabis products to treat a whole range of different conditions, but the two that stand out are chronic pain and severe childhood epilepsy.

In 2018, medical cannabis made the headlines in the UK thanks to Billy Caldwell, a young boy who suffered up to 300 epileptic seizures a day. Unable to get cannabis on the NHS, his mother travelled to Canada and was later stopped at Heathrow Airport and had the cannabis oil she was carrying removed.

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Since then, the National Institute for Health and Care Excellence (NICE) has taken a look at the research and on its website lists the conditions where it believes medically prescribed cannabis products might be helpful.

Illustration of a hand dropping a droplet of CBD oil on a fried breakfast © Jason Raish
© Jason Raish

These conditions are chemotherapy-induced nausea and vomiting, chronic pain (as part of a clinical trial), moderate to severe muscle spasms caused by multiple sclerosis, and severe childhood epilepsy that has not responded to other treatments.

Despite this, and even though it is legal in the UK for specialist doctors to prescribe cannabis oil for medicinal use, it is rare for patients to get such treatments on the NHS.

Using cannabis in something like shampoo is far less contentious, but also less likely to be effective. The reason you might want to add it to something like shampoo is because one the main active ingredients in cannabis, cannabidiol (CBD), has been shown to act on cannabinoid receptors in the body to help reduce inflammation.

This means it has the potential, if you rub it on your scalp, to improve things like dandruff, eczema and an itchy, flaky scalp. Whether it does is another matter.

When I checked out the website of a leading brand they were careful to point out that there is no evidence that a CBD-based shampoo will help your hair regrow or that “CBD shampoo can treat, prevent or cure any hair or skin conditions you may have”. Though they do add that it may help dandruff and “can maintain moisture levels within the hair”.

I think I will stick to my ordinary brand.

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Dr Michael Mosley is former medical doctor, health writer and BBC presenter. He’s best known as presenter of Trust Me I’m a Doctor on BBC Two but has also written a number of bestsellers about personal health and medicine, including The Fast Diet, Fast Asleep and Fast Exercise.