The surprising ways marijuana affects your body

Medically reviewed by Dr Roger Henderson, words by Annie Hayes
·6-min read
Photo credit: Shannon Fagan
Photo credit: Shannon Fagan

From Netdoctor

Cannabis, marijuana, hash, weed… there are a variety of different ways to refer to the leaves, stems and flower buds of the Cannabis sativa plant, which is typically smoked, vaped or consumed in food for its mind-altering effects.

The effects of weed vary from person to person, depending on how it’s consumed, how much you use, and how often you use it.

While mostly consumed recreationally – it’s the UK’s most widely-used illegal drug – medicinal marijuana is also available on prescription for certain medical symptoms and conditions.

The drug can cause both immediate and long-term effects, whether used legally or illegally. Here, pharmacist Navid Sole explains how cannabis interacts with the brain, and talks us through the effects of weed on different parts of the body:

The effects of weed on your brain and body

The most prominent psychoactive compound in cannabis is Tetrahydrocannabinol (THC). It’s one of at least 113 cannabinoids identified in cannabis, and is responsible for the feeling of being ‘high’. ‘THC is shown to affect a specific set of sites in the brain – the hippocampus, cerebellum and the basal ganglia,’ says Sole.

Each of these parts of the brain are linked to Endocannabinoid System (EC), which exists and is active in your body, even if you don’t use cannabis. Endocannabinoid receptors are an important part of this system, and are found throughout your body. When endocannabinoids – molecules made by your body – bind to them, it signals to the EC that it needs to take action.

‘When someone smokes marijuana, THC gets into the brain rapidly and attaches to these receptors,’ says Sole. ‘The EC system is finely tuned to react appropriately to incoming information. But THC overwhelms the EC system. It prevents the natural chemicals from doing their job properly and throws the whole system off-balance.’

Research has linked the EC to a variety of bodily functions, from inflammation levels to liver function, metabolism to learning and memory. This is why, when THC binds to these receptors, it triggers a cascade of effects that can influence your bodily function.

Effects of weed on your brain

The most noticeable effects of weed occur due to changes in the brain and central nervous system. When you ingest cannabis, the THC ‘triggers your brain to release large amounts of the feel-good hormone, dopamine, making you feel happy,’ says Sole. This can heighten your senses – for example, colours might appear brighter, or sounds may seem louder – and also warp your sense of time.

In the hippocampus, which is responsible for your memory, THC changes the way you process information. It can make it harder to focus, learn, and remember things. This also makes it harder to create new memories, Sole says. Typically, these effects only last for 24 hours or so after you stop ingesting cannabis.

However, the drug can harm developing brains. Using cannabis heavily in your teenage years may cause these effects permanently. Similarly, babies whose mothers use cannabis during pregnancy may develop issues with memory and concentration.

Marijuana is thought to temporarily ease chronic pain and inflammation. It has been shown to help control epileptic seizures, and relax stiff muscles and reduce spasms in people with multiple sclerosis. However, it also impairs brain areas that play roles in movement. ‘Changes also take place in the cerebellum and basal ganglia, which means that ingesting cannabis can alter your balance, coordination, and reflexes,’ says Sole.

Effects of weed on your lungs

The most common way to ingest cannabis is to smoke it, either with tobacco – for which the associated health risks are widely known – or on its own. ‘Similar to tobacco, marijuana smoke comprises a variety of toxic chemicals, which are an irritant to your throat and lungs,’ says Sole. These include ammonia and hydrogen cyanide, which can cause inflammation.

Photo credit: Eric O'Connell
Photo credit: Eric O'Connell

Cannabis may also damage your immune system, making you more susceptible to other illnesses. ‘Those who smoke marijuana regularly will experience wheezing, coughing and produce an increased quantity of phlegm – as well as being at high risk of bronchitis and lung infections,’ he continues. Cannabis smoke also contains carcinogens, so it may increase your risk of lung cancer, though research into this is ongoing.

Effects of weed on your blood

When you use cannabis, the THC enters your bloodstream and begins to circulate around your body. This process occurs quickly when you smoke or vape weed, and slowly when you consume it in food – for example, in a brownie. When this occurs, your heart rate speeds up by as much as 20 to 50 beats per minute, says Sole, and can last as long as three hours.

A meta-analysis of the health records of more than 20 million US patients aged 18 to 55 found that those who used cannabis were 26 per cent more likely to have a stroke than those who abstained. They were also 10 per cent more likely to have developed heart failure.

One of the most obvious – and frequently stereotyped – effects of weed are the bloodshot eyes. This is caused by blood vessels in the eyes expanding. But this isn’t necessarily a bad thing. In fact, by reducing blood pressure in the eyes, cannabis may temporarily ease symptoms of glaucoma, but research into this is ongoing.

Furthermore, scientists are looking into positive associations between cannabis, the circulatory system, and cancer. Scientists believe the drug may help stop the growth of blood vessels that feed cancerous tumours, however more research is needed before any claims can be made.

Effects of weed on your gut

Cannabis stimulates the appetite, leading to the phenomenon known as ‘the munchies’. In some instances, this is a welcome side-effect – for example, it can help people undergoing treatment for AIDS, cancer and other illnesses to regain weight.

‘Marijuana has also been used to ease symptoms of nausea or upset stomach,’ says Sole. People who are being treated with chemotherapy are sometimes prescribed cannabis to reduce side effects such as nausea and vomiting. However, when consumed in food, cannabis can cause digestive issues such as nausea and vomiting, due to how it’s processed in the liver, Sole adds.

Effects of weed on your mental health

One of the most concerning effects of weed are its implications for mental health issues. While many people use cannabis for its ‘mellow’ feeling, for some it can bring about the opposite effect, causing agitation, insomnia and irritability. Heavy, prolonged use can cause paranoia and has been associated with mental health disorders, such as depression and anxiety. In high concentrations, THC can cause you to hallucinate or lose touch with reality.

If you use cannabis and want to quit, your GP can refer you to services that can help. Alternatively, for friendly, confidential advice, contact Talk to Frank on 0300 123 6600. The helpline is available 24 hours a day, seven days a week.

Last updated: 13-10-2020

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