Commonly Abused Drugs: Opioids, Cocaine and Cannabis
Commonly abused drugs are substances people take repeatedly for their effects on the brain, even though they harm health. Three major classes studied in biology are opioids, cocaine, and cannabis. Each acts on the nervous system differently but all can lead to dependence.
Studying these drugs matters because misuse is a major public health problem affecting families and communities. Understanding their sources, actions, and risks helps students make informed choices and recognise why medical use must be strictly controlled.
Key Concepts
Opioids are obtained from the opium poppy Papaver somniferum. Morphine is extracted from latex; heroin (diacetylmorphine) is chemically derived from morphine. Opioids bind to opioid receptors in the brain and gut, reducing pain and producing euphoria but also slowing breathing.
Cocaine comes from the coca plant Erythroxylum coca. It blocks reuptake of the neurotransmitter dopamine in synapses, causing intense stimulation, increased heart rate, and a sense of energy. Crack cocaine is a smokable form.
Cannabis comes from Cannabis sativa. Its active compounds, cannabinoids like THC, bind to cannabinoid receptors in the brain, altering perception, mood, and coordination. Products include marijuana (dried leaves/flowers), hashish, and bhang.
- Dependence: the body adapts so normal function needs the drug.
- Tolerance: needing larger doses for the same effect.
- Withdrawal: unpleasant symptoms when the drug is stopped.
How It Works
- The drug enters the bloodstream and reaches the brain within seconds to minutes.
- It alters neurotransmitter levels — opioids mimic endorphins, cocaine traps dopamine, THC activates cannabinoid receptors.
- The reward circuit is overstimulated, reinforcing repeated use.
- With chronic use, natural neurotransmitter production falls, causing tolerance and dependence.
- Stopping suddenly triggers withdrawal, driving further use to avoid discomfort.
Worked Example
Compare dopamine action. Normally, after dopamine is released into a synapse, transporter proteins recycle about 80 percent within milliseconds. Cocaine blocks these transporters. If baseline synaptic dopamine is taken as 100 units, blocking reuptake can raise it to 300-400 units within minutes, prolonging stimulation. This three- to four-fold rise explains the intense rush and also the crash when dopamine is finally depleted, leaving the user feeling depressed and craving more.
Common Misconceptions
- Misconception: Cannabis is completely harmless because it is natural. Correction: Regular use impairs memory, coordination, and driving ability, and increases risk of mental health problems in vulnerable adolescents; natural does not mean safe.
- Misconception: Heroin is a medicine so it must be safe recreationally. Correction: Medical morphine is dose-controlled and monitored; street heroin purity varies, making overdose likely.
- Misconception: Cocaine gives real energy. Correction: It only borrows energy by overstimulating the nervous system, followed by exhaustion, paranoia, and cardiovascular strain.
Real-World Applications
Morphine remains essential for severe pain relief in surgery and cancer care under strict prescription. Cocaine was once used as a local anaesthetic but is now largely replaced by safer drugs like lidocaine. Cannabis-based medicines are being researched for epilepsy and chemotherapy nausea, but only purified, regulated preparations. Law enforcement and rehabilitation programs use knowledge of these drugs' chemistry to detect them in blood/urine tests and to design de-addiction therapies.
Opioids depress the nervous system, cocaine stimulates it by trapping dopamine, and cannabis alters perception via cannabinoid receptors — all three carry risks of tolerance, dependence, and serious harm when abused. Medical value exists only under careful control.