Drugs 15
CHAPTER 15
A drug is any substance taken into the body that modifies or affects chemical reactions in the
body.
There are many drugs, affecting different parts of the body. Some drugs act directly on the
nervous system. These are often grouped according to the effect they have, for example:
Stimulants promote (speed up) the action of the nervous system. Stimulants usually make the
drug user feel more confident and alert, and include amphetamines, caffeine and nicotine
(Nicotine is taken in tobacco smoke).
Depressants inhibit (slow down) the action of some part of the nervous system. The user feels
sleepy and less anxious, but may become dependent on the drug. Examples of depressants are
barbiturates, alcohol and cannabis.
Narcotics act like depressants but particularly target the brain. They work as pain-killers and may
bring about a feeling of drowsy well-being or euphoria. Narcotics such as heroin are very likely to
bring about drug dependence in the user.
Analgesics are mild pain-killers. Aspirin and paracetamol are widely used analgesics.
Medicinal Drugs
Drugs
Misused Drugs
(Recreational
Drugs)
Medicinal drugs
Antibiotics
1. Antibiotics are substances which kill bacteria, but do not harm other living cells or viruses.
2. Most Antibiotics are made by fungi but now a days some synthetic ones are also available.
Antibiotics and viral diseases
Antibiotics are not effective against viral diseases. This is because antibiotics work by disrupting
structures in bacteria such as cell walls and membranes, or processes associated with protein
synthesis and replication of DNA. Viruses have totally different characteristics to bacteria, so
antibiotics do not affect them.
Development of antibiotic resistant bacteria
If a course of antibiotics is not completed, some of the bacteria it is being used to destroy will not
be killed, but will have been exposed to the drug. Some of the survivors may be drug-resistant
mutants. When they reproduce, all their offspring will have the drug resistance, so the antibiotic
will become less effective. Some of them become resistant by transfer of genetic material
(plasmids) from friendly bacteria in our body. This resistance can come from wrong use of
antibiotics in viral infections.
IGCSE Biology
Dr. Muhammad Avais Chauhdry
1
Drugs 15
Misused drugs
Heroin
Opium poppies produce a substance called opium. Heroin, morphine and codeine belong to a
group of drugs called narcotics, made from opium.
Heroin acts a as powerful depressant (slowing down many functions of the brain) it relieves severe
pain, slows down breathing and functions of hypothalamus and produce short-lived feelings of
wellbeing (euphoria) and freedom from anxiety.
It can both lead to tolerance and physical dependence within weeks, so prescribed with caution (a
warning against danger), to patients in severe pain. The illegal use of heroin has terrible effects on
the unfortunate addict.
The overwhelming dependence on the drug leads many addicts into prostitution and crime to
obtain the money to buy it.
Endorphins
Endorphins are neurotransmitters, chemicals that pass along signals from one neuron to the
next. Endorphins help to reduce sensations of pain, affect mood and reduce sensations of hunger
and thirst. When heroin enters the brain, heroin is metabolized to morphine. Morphine molecule fit
into some of the endorphin receptors. Therefore, heroin makes people feel good.
Heroin can reduce the production of natural endorphins and affects the brain’s production of other
important neurotransmitters.
Withdrawal Symptoms of Heroin
There are severe withdrawal symptoms when an addict tries to give up the drug abruptly.
These symptoms are called going ‘cold turkey’ and can include anxiety, muscle aches, sweating,
abdominal cramping, diarrhoea, nausea and vomiting.
A ‘cure’ is a long and often unsuccessful process.
Additional hazards are that blood poisoning, hepatitis and AIDS may result from the use of
unsterilised needles when injecting the drug.
IGCSE Biology
Dr. Muhammad Avais Chauhdry
2
Drugs 15
Smoking
What is in tobacco smoke?
Tobacco smoke is a cocktail of up to 4000 different chemicals and is released into
the atmosphere in two ways:
Mainstream (Active smokers) smoke from the filter or mouth end of the cigarette/cigar/pipe
Side stream (Passive smokers) smoke from the burning tip of the cigarette/cigar/pipe tobacco.
Around 85% of tobacco smoke in a room will be side stream smoke - the form which contains
higher concentrations of many toxins than mainstream smoke. Breathing in this smoke is called
passive smoking and presents a health hazard to people nearby who inhale it. Of the
thousands of chemicals in tobacco smoke, three important ones are:
Carbon monoxide - a poisonous gas also found in car exhaust fumes. It combines with
haemoglobin and forms carboxyhaemoglobin in an irreversible reaction, preventing it from
transporting respiratory gases. This decreases the overall oxygen carrying capacity of a person.
Nicotine - a poisonous alkaloid drug that is addictive. 60mg of nicotine placed on the tongue
would kill an individual within minutes. It is absorbed by the body very rapidly, reaching the brain in
less than 30 seconds.
Addiction – once your body has got used to it, it is very hard to do without it. It is stimulant (makes
you more alert and active) raises blood pressure and heart rate, increases cholesterol deposition
in arteries.
It is also a stimulant which makes the heart beat faster and at the same time makes blood
vessels narrow, together these two effects raise blood pressure (causing long-term damage to
the circulation). The increased heart rate increases the demand for oxygen, but carbon monoxide
reduces oxygen availability - so heart muscle is more likely to be damaged.
TAR - a sticky, brown substance responsible for the staining of the fingers and teeth of smokers. It
appears in tobacco smoke as minute droplets.
It contains 16 carcinogenic substances hence it can cause lung cancer. Irritates the lining of the
trachea causing the increased production of mucus hence paralyses the cilia. It can also cause
bronchitis. Excessive coughing can breakdown the thin walls of alveoli decreasing the surface
area of gaseous exchange and this is called emphysema.
Other irritants in tobacco smoke include smoke particles, ammonia and sulfur dioxide.
1. Bronchitis: this is a disease caused by cigarette smoke which paralyses the cilia, as a
result filtration of dust particles will not be efficient. Excess mucus can reduce the diameter
of the tract and can get into the alveoli.
2. Lung Cancer: Cigarette smoke contains tar which has almost 16 carcinogenic substances.
Once the tar gets deposited in the wall of the alveoli, it can cause lung cancer.
3. Emphysema: results when the walls of the air sacs are destroyed. This happens because
smoke affects white blood cells which then destroy lung tissue. When the walls break down
there is less surface for gas exchange, and breathing becomes very difficult. Somebody
with emphysema may only be able to walk 2 or 3 meters before becoming breathless.
Emphysema is almost unknown in non-smokers.
IGCSE Biology
Dr. Muhammad Avais Chauhdry
3
Drugs 15
Alcohol
Small amounts give a sense of wellbeing, with a release from anxiety. Alcohol is quickly absorbed
through the wall of the stomach, and carried all over the body in the blood. It is eventually broken
down by the liver, but this takes quite a long time.
Drinking fairly small quantities of alcohol is not dangerous, but alcohol does have many effects on
the body which can be very dangerous if care is not taken.
Alcohol lengthens reaction time.
The alcohol in wines, beer and spirits is a depressant of the central nervous system which means
that even small amounts of alcohol slow down the actions of parts of the brain, so alcohol
lengthens the time you take to respond to stimuli.
Alcohol can increase aggression in some people.
Alcohol reduces inhibitions because it depresses that part of the brain which causes shyness. This
may be considered an advantage in ‘breaking the ice’ at parties. But it can also lead to
irresponsible behavior such as vandalism and aggression. Research has shown that at least 50%
of violence in the home in many countries is related to drunkenness, and that alcohol has played a
part in the criminal behavior of around 60% of people in prison in western countries.
Alcohol has widespread effects throughout the body
1. Cardiovascular effects include anemia (alcohol poisons bone marrow) and increased deposits
of fat in the coronary arteries.
2. Skin blood vessels dilate so warm blood flows to the skin. The body feels warm and
comfortable, but hypothermia may result at low environmental temperatures.
3. Sex organs are stimulated but do not work well. Sperm count may be reduced.
4. Intestines are irritated, causing indigestion, nausea, diarrhoea and ulcers.
5. In pregnant women, alcohol may cross the placenta to the unborn child. The child may develop
addiction slowly, especially its nervous system (this is called fetal alcohol syndrome).
6. Liver cells work harder to detoxify the alcohol. Cells lining blood vessels are damaged causing
cirrhosis - liver function begins to fail.
7. Kidneys cannot reabsorb water very well. Too much water is excreted and the body becomes
dehydrated (this is responsible for the headache following a drinking bout).
8. Cancer of the tongue and esophagus are much more likely in heavy drinkers (especially if they
also smoke).
IGCSE Biology
Dr. Muhammad Avais Chauhdry
4