Since a change in the law in November 2018, some cannabis-based medicines have been available on prescription. However, there is only evidence of benefit for treating a small number of conditions.
Many cannabis-based products are also available to buy online, without a prescription, but the quality and content of these products is not known. Most of these products, including those called 'CBD oils', are illegal to possess or supply. Also the exact quality and content of these products is usually not clear and they may not be safe to use.
Some health stores also sell certain types of 'pure cannabidiol (CBD)'. However, they tend to only contain very small amounts of CBD, so there is little evidence of any benefit, and there's no guarantee these products will be of good quality.
What are cannabis-based medicines?
Cannabis-based medicines (CBMs) are medicines derived from cannabis that have been used for treating medical conditions. CBMs contain cannabinoids derived from the cannabis plant, including delta-9-tetrahydrocannabinol (THC), cannabidiol (CBD), or a combination of THC and CBD. THC is the constituent of cannabis that causes the 'high', whereas CBD is not intoxicating at typical doses. THC is more likely than CBD to cause side-effects.
Man-made (synthetic) cannabinoids are also available. They mimic the effects of specific cannabinoids such as THC.
What conditions can cannabis-based medicines be used to treat?
CBMs have been studied in a variety of different conditions but there is currently only evidence of benefit in a few conditions. Therefore only a few people are likely to get a prescription for medical cannabis. Currently, CBMs are only prescribed for the following conditions:
- Adults with nausea or vomiting caused by chemotherapy.
- People with muscle stiffness and spasms caused by multiple sclerosis (MS).
- Children and adults with rare, severe forms of epilepsy.
There is currently no evidence of benefit using CBMs for any other condition such as chronic pain or fibromyalgia.
Nausea and vomiting caused by chemotherapy
Chemotherapy may cause a person to feel sick (nauseous) or vomit. Nabilone is a man-made (synthetic) CBM that can be prescribed by a specialist to help relieve these symptoms, but only when other treatments have not helped or are not suitable.
Muscle stiffness and spasms caused by multiple sclerosis
Nabiximols (Sativex®) is a CBM that is sprayed into the mouth. In the UK, it is licensed for people with MS-related muscle spasticity that has not improved with other treatments.
Severe treatment-resistant epilepsy
Epidyolex® is a highly purified liquid containing cannabidiol (CBD). It will not get you high, because it does not contain tetrahydrocannabinol (THC), the chemical in cannabis that makes you high. Epidyolex® can be prescribed for patients with Lennox-Gastaut syndrome or Dravet syndrome, which are both rare types of epilepsy.
What are the side-effects of cannabis-based medicines?
The risks of using CBMs containing THC (the chemical that gets you high) are not currently clear. Ongoing clinical trials are needed before they can be used safely. 'Pure' products that only contain CBD, such as Epidyolex®, do not carry these unknown risks linked with THC. However, most products will contain a certain amount of THC. The main risks of THC cannabis products are:
- An increased risk of a psychotic illness such as schizophrenia.
- Dependency on the medicine (addiction). This risk is probably small when its use is controlled and monitored by a specialist doctor. The risk increases with increasing levels of THC.
Cannabis bought illegally off the street, with unknown quality, ingredients and strength, is the most dangerous form to use.
The other possible side effects of CBMs are:
- Poor appetite
- Feeling sick
- Change in behaviour or mood
- Feeling very tired
- Feeling 'high'
- Suicidal thoughts
Like many other medicines, CBMs can also affect how other medicines work. Always discuss possible interactions with a specialist. CBD can also affect how your liver works, so doctors would need to monitor you regularly.
How can you get a prescription for a cannabis-based medicine?
You cannot get a prescription for a CBM from a GP. A CBM can only be prescribed by a specialist hospital doctor. The specialist will advise trying other treatment options first, before considering a cannabis-based product. A prescription for medical cannabis would only be given when it was believed to be in your best interests, and when other treatments had not worked or were not suitable.
After the initial prescription, further prescriptions for CBMs may be issued by another doctor as part of a shared care agreement under the direction of the initial specialist prescriber.
What else do you need to know?
Before being prescribed a CBM, the following should be discussed with you:
- The potential benefits and harms, including any risk of dependence or interaction with other medicines.
- The licensing status of the medicines.
- How long you might take the medicine.
- How long it will take to work.
- What it has been prescribed for and how to take it.
- How it may affect your ability to drive (see Further Reading below for the advice from the Department of Transport on drug driving and medicine.
- The need to seek advice before travelling abroad about the legality of cannabis-based medicinal products in other countries (see the UK Government's advice on travelling with medicine containing a controlled drug in the Further Reading section below).
- The importance of not allowing others to use the prescribed medicine.
Further reading and references
Cannabis-based medicinal products; NICE Guidance (November 2019 - last updated March 2021)
Cannabidiol with clobazam for treating seizures associated with Lennox-Gastaut syndrome; NICE Technology Appraisal Guidance, December 2019
Cannabidiol with clobazam for treating seizures associated with Dravet syndrome; NICE Technology Appraisal Guidance, December 2019
Walitt B, Klose P, Fitzcharles MA, et al; Cannabinoids for fibromyalgia. Cochrane Database Syst Rev. 2016 Jul 187:CD011694. doi: 10.1002/14651858.CD011694.pub2.
Mucke M, Phillips T, Radbruch L, et al; Cannabis-based medicines for chronic neuropathic pain in adults. Cochrane Database Syst Rev. 2018 Mar 73:CD012182. doi: 10.1002/14651858.CD012182.pub2.
Chesney E, McGuire P, Freeman TP, et al; Lack of evidence for the effectiveness or safety of over-the-counter cannabidiol products. Ther Adv Psychopharmacol. 2020 Sep 910:2045125320954992. doi: 10.1177/2045125320954992. eCollection 2020.
Drug driving and medicine: advice for healthcare professionals; Department for Transport, July 2014