Drugs

What counts as a drug

Cambridge defines a drug as any substance taken into the body that modifies or affects chemical reactions in the body. The definition is deliberately wide. It covers medicines such as antibiotics and painkillers, but also alcohol, nicotine, caffeine and illegal substances. What makes something a drug is not whether it is legal or helpful - it is that it changes the chemical reactions going on inside you.

Because the definition is so broad, examiners expect you to apply it. Paracetamol, the caffeine in a cup of coffee, the alcohol in a lager and heroin are all drugs, because each one alters the body's chemistry in some way. A glass of water is not a drug: it does not modify chemical reactions, it simply replaces water that has been lost.

Notice that the word drug is neutral. Some drugs treat disease, some are taken for pleasure and some cause serious harm, but all of them fit the same definition.

• Medicinal drugs: antibiotics, painkillers and anaesthetics, taken to treat or prevent disease.
• Recreational drugs: alcohol, nicotine, caffeine, heroin and ecstasy, taken for their effect on mood or perception.
• The legal status of a drug does not change whether it is a drug - it changes the consequences of using it.

Antibiotics: what they can and cannot do

Antibiotics are drugs that kill bacteria, and they are used to treat bacterial infections such as certain throat, chest, skin and urinary infections. They work because bacterial cells have structures and processes that human cells do not - for example bacteria build a cell wall and make their own proteins - so an antibiotic can attack the bacterium without harming your own cells.

Antibiotics do not affect viruses. A virus is not a cell: it has no cell wall, no bacterial enzymes and no metabolism of its own, so there is nothing for an antibiotic to attack. This is why antibiotics are useless against viral infections such as the common cold and influenza (flu), and why a doctor should not prescribe them for those illnesses.

Different antibiotics work on different bacteria, so the doctor has to choose one that is effective against the bacterium causing that particular infection. A broad antibiotic given for the wrong infection will not help and will add to the resistance problem.

Antibiotic resistance and MRSA

Antibiotic resistance means that a bacterium is no longer killed by an antibiotic that used to work against it. Resistance spreads because of natural selection. Within a population of bacteria, random mutation can produce a few individuals that are not affected by the antibiotic.

When the antibiotic is used, it kills the non-resistant bacteria but leaves the resistant ones alive. Those survivors reproduce rapidly, passing the resistance on to their offspring, so the proportion of resistant bacteria in the population rises. The antibiotic then becomes less effective, and the more often it is used, the faster this happens.

Two human behaviours make the problem worse: overuse, such as prescribing antibiotics for a viral illness when they cannot help, and patients not completing a full course, stopping as soon as they feel better, which leaves the tougher bacteria alive to multiply.

MRSA (meticillin-resistant Staphylococcus aureus) is a well-known resistant bacterium. It is common in hospitals, where it can cause serious wound and bloodstream infections because many of the usual antibiotics no longer kill it.

Painkillers: treating the symptom, not the cause

Painkillers are drugs that relieve symptoms such as pain, fever and inflammation. They do not kill the pathogen causing the illness, so they do not cure the infection - they make the patient more comfortable while the body's immune system deals with it.

Common examples are paracetamol, aspirin and ibuprofen. If you have a bacterial infection you may be given a painkiller and an antibiotic: the antibiotic kills the bacteria, while the painkiller eases the pain and brings down the fever.

This distinction matters in exams. An antibiotic attacks the cause of the infection, the bacterium; a painkiller only relieves the symptom. Writing that a painkiller kills the bacteria will lose the mark.

How drugs affect the nervous system at the synapse

Many drugs work by changing transmission at a synapse, the junction between two neurones. Normally an electrical impulse arriving at the end of a neurone causes neurotransmitter molecules to be released across the synaptic gap; they bind to receptors on the next neurone and start a new impulse there. A drug can slow this process, speed it up or distort it.

Depressants slow down nervous impulses, so they reduce the speed of reactions and the activity of the brain. Alcohol and heroin are depressants. They can make a person drowsy, slow their reactions and impair coordination and judgement.

Stimulants speed up nervous impulses, increasing alertness and making the heart beat faster. Nicotine, caffeine and ecstasy are stimulants.

Hallucinogens distort the senses, so the user sees, hears or feels things that are not there. LSD is an example. They change perception rather than simply speeding the nervous system up or slowing it down.

Alcohol and heroin: the specific harms

Alcohol is a depressant and is legal, but misusing it causes serious harm. It slows nervous impulses, which impairs judgement, reaction time and coordination - a major reason why drinking and driving is so dangerous and illegal. Long-term heavy drinking damages the liver, causing cirrhosis, in which healthy liver tissue is replaced by scar tissue. Alcohol also crosses the placenta, so it can damage a developing foetus and cause lifelong problems; no safe level of alcohol in pregnancy is known.

Heroin is a very addictive depressant. Because the body adapts to it, the user needs more and more to get the same effect, and stopping causes withdrawal - unpleasant and sometimes dangerous symptoms such as anxiety, sweating, muscle pain and vomiting. Many users inject, and sharing needles spreads infections such as HIV and hepatitis B and C, as well as causing abscesses and damaged veins. Heroin also suppresses the immune system, so users catch other infections more easily.

Social and legal consequences, and reducing the harm

Drug misuse has consequences beyond the body. Socially, it can break up families, cost people their jobs and lead to crime, debt and homelessness. Legally, supplying or possessing illegal drugs is a criminal offence and can lead to a fine, a prison sentence and a criminal record; driving while over the alcohol limit is also illegal. The exam asks about the effects of drug misuse, not only its biology, so these consequences are worth remembering.

Some harms can be reduced. Needle-exchange programmes give injecting drug users sterile needles and syringes, which cuts the spread of HIV and hepatitis between users without encouraging them to inject. Antibiotic resistance can be slowed by using antibiotics only when essential - prescribing them only for bacterial infections and taking the full course exactly as directed.

Using antibiotics only when essential means fewer bacteria meet the antibiotic, so fewer resistant mutants are selected and given the chance to multiply and spread. This keeps the antibiotic effective for the patients who really need it.