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Studies have found that THC, the principal psychoactive substance within the cannabis plant, can cause cognitive defects, particularly if taken long term. Recent medical evidence strongly suggests that the long-term use of cannabis by people who begin use at an early age—they display a higher tendency towards mental health problems and other physical and developmental disorders. According to a 2014 Northwestern Medicine study, when cannabis use begins in the teen years, it can have a significant impact on brain development, including decreased brain activity, fewer neural fibres in certain areas and a smaller than average hippocampus, which controls learning and memory functions.

In my years working in the prison service, I witnessed first-hand the harms associated with smoking cannabis long term. Many of the young people suffered from paranoia, anxiety, memory problems and other such problems. We therefore need further research into the long-term use of cannabis, particularly its effects on young people, before we can say it's safe to reclassify it for medical use.

The MS Society state that there is currently insufficient,

'evidence to suggest that smoking cannabis can treat spasticity or pain in MS and smoking can negatively impact on an individual’s MS.'

Therefore, we need to focus on extracting the benefits from the cannabis plant, whilst, at the same time, mitigating the potential harms. The main problem with cannabis is the complexity of the active ingredients, the huge variation that exists in the levels of compounds between the species of plants, the various cultivars, and even in plant to plant. While a particular strain of plant, or cultivar, could be developed to increase the medical benefits and limit the psychoactive effects, there is no guarantee that the mix of active ingredients are the same from one crop to the next. There is no disputing—. [Interruption.] Not another one, sorry. I'm out of time.

There is no disputing the huge benefits that the various cannabinoids can have in treating a number of conditions. I believe we have to work at isolating those compounds. In the UK, we license Sativex, an extract of cannabis, to treat MS symptoms. In the US, too, additional cannabinoids have been created to treat the side effects of chemotherapy. I hope to see this work accelerated.

Given the possible harms and the limited evidence base about the efficacy of smoking cannabis for medical purposes, I cannot support the legalisation of medical cannabis at this stage. I will, therefore, be abstaining on the motion today, although other Members in my party will have a free vote. I look forward to new research into the medical use of cannabis, and if it can be shown that the benefits vastly outweigh potential harms, I would be happy to support the rescheduling of the drug at that stage. Diolch yn fawr.