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How it works in the brain

Cannabis (skunk, marijuana, pot, grass, weed, joints, spliff, hashish, blow) is a hallucinogen, a depressant and our commonest illegal drug.

Brain cell and the effects of cannabis

100 billion nerve cells (neurons) may have up to 10,000 connections to other neurons in the vast brain network. Messages pass along the nerve fibres as electrical impulses, then cross the gap between the neurons (the synapse) in the form of chemicals – neurotransmitters – the brain’s natural drugs. Each neurotransmitter molecule has a particular shape to fit into its receptor site on the next neuron as a key fits into a lock.

Brain neurone and cannabis

Mind-altering drugs like cannabis or more specifically THC (tetrahydrocannabinol), the ingredient that gives the “high”, mimic the shape of these neurotransmitters so the brain is “fooled”. Cannabis mimics anandamide and also interferes with the transmission of the other neurotransmitters because THC dissolves in the fatty cell membranes and persists. Fifty per cent of the THC is still there after a week and ten per cent a month later. Traces are still detectable in hair and urine for weeks after that.


Old herbal cannabis (1960s to 70s) had a THC content of 1-2% - unavailable now. In 2008 (Home Office Potency Study), hash (resin) had an average of 5.9%THC and 3.5% CBD (cannabidiol). Skunk 16.2% THC and only 0.1% CBD occupied 80.8% of the market. The anti-psychotic CBD in hash helped to counteract the psychotic effects of THC in hash.

The Immediate effects

Taken for euphoria (a ‘high’), usually smoked with/without tobacco, or eaten in “hash” cakes. It is more difficult to control dosage with food. Effects are delayed and children in Colorado (where there islegalised cannabis) have been admitted to hospital with poisoning.  Enhances the mood you’re in, you may well feel worse. Some people get hungry - ‘the munchies’ or start giggling.
An intoxicant, like alcohol, so people should not drive. If you have a joint today you should not be driving tomorrow. (Airline pilots on flight simulators could not ‘land’ their planes properly even 24 hours and more after a joint and had no idea anything was wrong). An average joint has the same effect as being just over the legal alcohol-driving limit. The combination of cannabis and alcohol is 16 times more dangerous when driving than taking either drug alone. Panic attacks, anxiety, hallucinations and paranoia can occur immediately after a joint. There is a feeling of time slowing down.

The long-term effects.

Just one joint a week or even once a month will ensure a permanent presence of THC in the brain. Since other neurotransmitters are affected, new nerve connections can’t be made properly. Concentration, learning and memory are all badly affected. Few children, using cannabis even occasionally, will achieve their full potential. A Study from birth in Dunedin found that the IQ of teens who get hooked and continue to use falls by average 8 points – permanently. The brains of children are still developing so the earlier they start using, the worse the damage. Children fail to mature properly, have fixed ideas, can't reason thing out, find relationships difficult, often drop out of education and are lonely and miserable.


Psychological addiction: Cannabis, like alcohol, heroin, cocaine etc, increases the amount of dopamine, the ‘pleasure’ neurotransmitter in the reward centre of the brain. However two other substances are produced at the same time. One dampens the effect so more is needed (tolerance) and the other persists and makes more connections and receptor sites, making the brain more sensitive. Strong cravings can last months, even years, and be triggered by just the sight of drug paraphernalia.

Physical addiction: As more and more THC is consumed, production of anandamide gradually ceases - it’s not needed. If THC is stopped the receptor sites are left empty but need filling. This results in withdrawal symptoms, irritability, anxiety, depression, lack of sleep etc. The withdrawal symptoms are not so dramatic as those of heroin as THC persists in the brain cells for some time. 10% of those who TRY cannabis will become addicted, in teenage users this rises to 1 in 6. There is no foolproof cure for any type of addiction.


Mental Health


THC reduces serotonin (the ‘happiness’ neurotransmitter) so users may suffer from depression. Cannabis psychosis has been reported in scientific papers since 1845. At London’s Institute of Psychiatry Professor Robin Murray has shown that anyone can become psychotic, it’s simply a matter of how much they take, and that skunk users are 7 times more likely to suffer
psychosis than those who use hash. He said,

Five years ago 95% of psychiatrists would have said that cannabis doesn’t cause psychosis. Now I would estimate that 95% say it does.

Cannabis (THC) increases the amount of the neurotransmitter dopamine in the brain. The brains of psychotics and schizophrenics have an excess of dopamine. Both conditions worsen with continued use of the drug. Scientists are generally in agreement that there is a hereditary basis for schizophrenia, the relevant gene(s) are being sought. Cannabis users double the chances of developing this condition. Schizophrenia is a chronic, often lifetime condition. It was reported (Murray et al, 2015) that users of skunk increase their risk of psychotic disorder Three-fold compared to those who have never used, daily skunk users – five-fold.   24% of first-episode psychosis in one South London area was attributable to skunk use.

Violence and Suicide

A Swedish study found more suicides among pot users than those who used alcohol, heroin or amphetamines. This may be due to depression. The manner of death was more violent. No other group jumped from high buildings. A study in 2001 from Dunedin found that young male cannabis users were nearly 4 times more likely to be violent than non users, the risk for alcohol users was around 3. Violence seems to be linked to the psychosis or the withdrawal from the drug.
Parents and siblings may be injured and homicides are not uncommon.

Other Effects on the body

Cannabis smoke deposits three to four times as much tar as tobacco smoke does in the lungs and airways. Cases of lung cancer, bronchitis, bullous lung disease, and emphysema have been reported. Rare head and neck cancers are now being found in young pot smokers. Asthma may worsen. The average age for these conditions in tobacco smokers is 64. Cancers in the head, neck, tongue and testicles have been reported.

THC interferes with the production of new cells being made in an adult body - white blood cells, sperm and foetal cells. It causes faulty copying of DNA, and hastens programmed cell death (apoptosis). The immune system is damaged. Fewer white blood cells are produced, many are abnormal and can't fight off infections. People are more vulnerable to disease, their illness is more severe and they stay sick longer. Sperm production is decreased. Infertility and even impotence have been reported.

Babies born to cannabis using mothers are smaller, hyperactive, have behaviour and learning problems and are 10 times more likely to develop one form of leukaemia. They are also likely to
have withdrawal symptoms and to use cannabis themselves when they reach adolescence.

There have been reports of miscarriages and ectopic pregnancies.
Blood pressure and heart rates rise to the levels of real stress. Heart attacks have been reported. Two teenagers had strokes and died after bingeing on cannabis, another was left paralysed.


Ongoing animal experiments in Sweden are finding that cannabis ‘primes’ the brain for the use of other drugs and The Christchurch Longitudinal study concluded that “the use of cannabis in late adolescence and early adulthood emerged as the strongest risk factor for later involvement in other illicit drug use”.

Medical Cannabis

Smoking cannabis for medication is like eating mouldy bread to get your penicillin, or chewing willow bark for aspirin. Medicines by law must be pure single substances so their actions are predictable and controllable. Synthetic THC (Nabilone, UK and Marinol, USA) have been available for many years to combat the nausea of chemotherapy and to stimulate appetite but have many side-effects. Sativex ( THC + CBD) has been deemed by some Health Authorities to be ineffective. The ‘scam’ was started in 1979 by a pot-using American lawyer who said, ‘We will use the medical marijuana argument as a red herring to give pot a good name’.

Brain Scans

In the last few years, scans have found brain damage in cannabis users. The volume of gray matter (brain cell bodies) has been reduced in some areas rich in cannabis receptors. These include the hippocampus (learning), amygdala (emotions), anterior cingulated (motivation) and prefrontal cortex (intelligence, problem solving and decision-making). A smaller white matter (nerve fibre) volume has been seen in the cerebellum, similar to that in the brains of schizophrenics.

don't want drugs?

Some ideas for excuses for not accepting or buying drugs:

  • I’m on medication (antibiotics, cough mixture etc), have bronchitis, asthma...
  • I’m in a football match tomorrow and don’t want to mess it up

more ideas for tricky situations


Dr Karen Randall: Marijuana & The Emergency Department

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