O Level Notes : Biology - Drugs

A drug is any externally administered chemical substance (other than food) that modifies or affects chemical reactions in the body.



A drug is any externally administered chemical substance (other than food) that modifies or affects chemical reactions in the body.

A drug may be beneficial to the body or harmful to it depending on the way how we use it. Usually, problems arise when people become addicted or dependant on a particular drug.

Drugs are consumed in many ways. They may be generally divided into three main categories:

  1. Medicinal drug.
  2. Socially-acceptable drug.
  • Drugs of abuse.


Several drugs are used under medical supervision to treat diseases, to relieve pains and in surgery. Penicillin is a medicinal drug which is used to treat bacterial diseases and Pauldrine, another drug is used to treat the malarial parasite.

  • Antibiotics:

Antibiotics are chemicals that are widely used to treat many infectious diseases caused by microorganisms. These chemicals are produced by chemicals and moulds. However, a few antibiotics are man-made.

Antibiotics do not harm viruses. Doctors sometimes prescribe antibiotics when you have a viral infection. This is actually a deterrent measure. It prevents you from acquiring a possible bacterial infection because the body resistance is very low.

There are 4 main types of antibiotics:

  1. Penicillins: they are made by the fungus penicillium. These attack a fairly narrow range of bacteria and are therefore called narrow-spectrum antibiotics,
  2. Cephalosporins: these are made by the mould cephalosporium. They are useful against bacteria which have developed resistance to penicillin.
  3. Tetracyclines: these are made by the bacterium streptomyces aureofaciens, which act against a variety of bacteria. Therefore they are called broad-spectrum antibiotics.
  4. Erythromycins: they work against the same sort of bacteria as penicillin, and are therefore useful against bacteria which have developed resistance to penicillin.

Antibiotics essentially work in two ways. Penicillin, for example, prevents the bacteria from making essential components of the cell wall which makes it easier for the body’s immune system to destroy it. Tetracyclines, on the other hand, damage the protein-producing machinery inside the cytoplasm of the bacterium, thereby preventing it from dividing. This inhibits the growth of the bacterium.


Anaesthetics are drugs that that make the body unable to feel pain. Cocaine, the first known local anaesthetic, causes a loss of sensation in the area in which it is injected.

Certain drugs can relieve pain without causing numbness or affecting consciousness. These drugs are called analgesics, e.g. aspirin which relieves minor pain and reduces fever.


An alcoholic beverage is a drink containing ethanol, commonly known as alcohol. Alcoholic beverages are divided into three general classes: beerswines, and spirits. They are legally consumed in most countries, and over 100 countries have laws regulating their production, sale, and consumption.


The regular intake of alcohol in high dosages, causes many long and short term effects on the various body parts, like the bone structures, bloodstream, liver, stomach, pancreas, heart, peripheral tissues and mouth. Regular or high consumption of alcohol can cause serious central nervous system disorders.

Short Term Effects

Alcohol affects the nervous system by stamping down the signals between the spinal cord, nerve system and the brain. The alcohol is absorbed by the blood which results into the slaking of the nerve tissues and they become totally numb when in this state of 'drunkenness'. At this time, the digestive system cannot digest the alcohol. This is the only difference between alcohol and any other sedative.

As we all know, there are two human body systems, namely - the voluntary body system and the involuntary body system. The voluntary system controls the movements of the muscles, while the involuntary system controls the speed of the body parts, the heart beats and the electrical signals that pass from the brain through neurons. The involuntary body system gets affected largely if someone is consuming excessive quantities of alcohol. Alcohol is most probably the biggest depressant of the central nervous system. It also enhances the activities of the 'gamma aminobutyric acid' (GABA), and weakens 'glutamine'. As a result the person's behavior gets torpid. Lack of coordination and dimming behavior are the basic effects seen, when a person is drunk.

Long Term Effects

The effects of alcohol on the central nervous system depend on the duration and the quantity of the alcohol consumption. They also vary from person to person on the basis of the tolerance capacity. These changes are not stable as they continuously keep on changing. The cells become semipermeable to alcohol, thicker in short. These unhealthy cells weaken the nervous system a lot. Also, the high tolerance level of a person to the alcohol, makes him more prone to various kinds of infections. Severe consequences like - heart attacks, brain strokes and dementia may also appear.

Chronic or gradual alcohol consumption leads to addiction of the alcoholic beverages. The typical symptoms of it may include panic, anxiety, tremors of body parts, nausea and sleep disorders. There could be some more withdrawal symptoms like hallucinations. The chronic drinking habit can damage both the frontal lobes, it also reduces the brain weight and the brain size.


Alcoholics are a liability to the society


  • They may neglect their work and families, and exhibit violent behavior especially towards family members.
  • Many crimes are committed by a person when he is under influence of alcohol.



Alcohol stimulates acid secretion in the stomach which increases the risk of gastric ulcers. Prolonged consumption of alcohol over an extended period of time may damage the liver, which may eventually lead to the cirrhosis of the liver – a disease in which the liver cells are destroyed and replaced by fibrous tissue making the liver less able to function. These problems caused by prolonged consumption of alcohol may cause death.


Drug abuse is the taking of drugs excessively, or not under a doctor’s prescription.

  • Effects of drug abuse:
  • Tolerance: it is the condition where a person has to keep on taking more and more of a drug to keep on achieving the same effect.
  • Addiction or dependence: where a person experiences withdrawal symptoms of he does not take the drug. Withdrawal symptoms may nclude the following:
    • Being physically ill (experiencing nausea, vomiting, uncontrolled trembling etc.)
    • being mentally disturbed (experiencing acute anxiety, depression etc.
  • Types of drugs:

drug abuse has become a very serious problem in some countries. Drugs can be sorted into many categories; and the same drug may fall in more than one categore. For example, alcohol can both be classifies as a socially acceptable drug and at the same time it can be abused. For convenience, drugs can be classified as follows:

  • Stimulant drugs:

These drugs stimulate the nervous system, e.g. cocaine and amphetamines. Amphetamines are used to counteract depression, prevent fatigue and counteract hunger in patients who are on a diet.


  • Depressant drugs:

These drugs include barbiturates which are used as sleeping pills. They are used to overcome insomnia and to treat epilepsy.

  • Hallucinogenic drugs:

                                         These drugs cause a person to experience illusion, hallucinations and distorted images. Examples include cannabis, also known as marijuana and LSD.


  • Opiates:

These drugs include opium, morphine and heroin. These are narcotic drugs because they relieve pain and induce sleep and stupor.



Heroin is a powerful depressant. Its abuse is one of the biggest addiction problems in many countries. It is several times powerful than morphine. Initially, it dulls the senses, giving the feeling of well-being. It relieves tension, reduces hunger and makes the user feel sleepy. One of its most dangerous effects include that user becomes rapidly addicted to it and needs an increasingly larger dose to keep on achieving the same effect. An addicted person is physically dependant on the drug. If the drug is withheld, user experiences withdrawal symptoms. These include anxiety, stomach upsets, sweating, goose pimples, watery eyes, vomiting, diarrhea, convulsions and hallucinations. In severe cases, death may occur.

Another problem related with the use of heroin is that addicts often directly inject the drug into their veins and often share needles.





Most people who smoke do so because they can't stop. Nicotine is a highly addictive substance that makes people feel energized and alert. Smokers get a rush after a cigarette, and giving up produces withdrawal symptoms that include difficulty sleeping and cravings. Seventy percent of people who quit smoking eventually start again.

Tobacco advertising also has a big influence on why people smoke. For years, the industry has focused on making smoking glamorous through advertising in movies, television, and billboards. While cigarette advertising is now controlled, its influence can still be felt in the form of free samples, smoking cartoons, and the promise of cool merchandise that can be obtained in exchange for coupons printed on cigarette packs.

Smoking also produces psychological dependency. Many people smoke because it helps them relax and cope with difficult situations, or because it gives them confidence. Others smoke when they feel bored. Smoking produces a feeling of satisfaction that's difficult to give up. Finally, people who smoke are usually in denial – they know that smoking is bad, but they convince themselves it's simply "not as terrible as they make it sound."

Many teenagers start smoking due to peer pressure. They may also smoke to feel more mature or as a form of rebellion against parental authority. It hasbeen proved that children are also more likely to smoke if their parents do.


Cigarette smoke contains more than 4000 chemicals; many of which are harmful to the body. We will now discuss about the ones that have been linked to health problems.



                         This is the addictive drug present in tobacco.


  • It stimulates the brain at first making the smoker feel alert, and relaxes the muscles. Later, it dulls the brain and senses.
  • It causes the release of adrenaline. This increases the rate of heart beat and blood pressure.
  • It makes blood clot easily. Therefore, it increases the risk of blood clots blocking blood vessels. If such clots block the coronary arteries, a heart attack may occur, or if they occur in the capillaries of the brain, a stroke may occur.


  • If the concentration of carbon monoxide in the air is increased by 1%, it can produce a deadly effect- a person inhaling such a high concentration of the gas may die within 10 minutes. This happens because carbon monoxide reduces the efficiency of the red blood cells to transport oxygen
  • It also increases the rate at which fatty substances are deposited on the inner walls of the arteries, causing their lumen to become narrower. This increases the risk of arthrosclerosis.
  • It damages the lining of blood vessels, thus increasing the tendency of blood to clot and so block the blood vessels.


  • TAR:

              This is a brown, sticky substance that accumulates in the lungs during smoking.


  • ■ It contains many cancer causing (carcinogenic) chemicals. Normally, cell division occurs continuously in the lungs to replace the membrane of the air sacs. Tar induces these cells to divide at an abnormal rate. Such uncontrolled multiplication of cells results in outgrowths or lumps of tissues (cancers). These block off the air sacs, reducing the efficiency for gaseous exchange.
  • ■ Tar paralyses the cilia lining the air passages. This prevents the cilia from removing the dust particles from the lungs and the trachea.



 These include hydrogen cyanide and acrolein.

  • These substances paralyze the cilia in the air passages and weaken the walls of the alveoli.
  • They irritate the cells lining the air passages causing them to produce mucus. his, in turn, causes a ‘smokers cough.’ The coughing bursts the weakened walls of the alveoli, causing a decrease in the wall area as many minute alveoli coalesce to form a reduced number of large alveoli. This process eventually causes the gaseous exchange surface in the lungs to become greatly reduced- a disease known as EMPHYSEMA.



Studies show that the more you smoke the greater your risk of getting lung cancer. If a person smokes 10 cigarettes per day, the risk of him getting lung cancer is 10 times greater and if he smokes 20 cigarettes, the risk is 20 times greater.


In the above picture, the lung on the left appears to be of normal color and the air sacs are too fine to be visible whereas the lung on the left has acquired a dark blackish color and the burst air sacs are clearly visible.


                                       Cancer is an uncontrolled abnormal division of cells producing outgrowths or lumps of tissues. Smoking increases the risk of cancers of the mouth, throat (larynx), pancreas, kidneys and urinary bladder.


                                                  In chronic bronchitis, the epithelium or membrane lining under the inner surfaces of the bronchi or main air passages to the lungs become inflamed and narrowed. Excessive mucus is secreted. This reduces the air flow in the air passages, and so breathing becomes difficult. The person coughs and wheezes persistently to clear his passages, making his lungs more likely to be infected by bacteria. It also results in an increased production of sputum or phlegm. Breathing irritating chemicals (those present in cigarette smoke) causes bronchitis.



                                Emphysema is commonly associated with chronic bronchitis and cigarette smoking. The partition walls between the alveoli between the alveoli break down because of intense coughing, enlarging the air spaces and decreasing the surface area of the lung. This reduces the absorption of oxygen. The lungs become over-inflated with air and lose their elasticity, making breathing difficult. An infected person spends a lot of his energy just to keep breathing, and he wheezes.




The carbon monoxide and nicotine in cigarette smoke affect the development of fetus. The carbon monoxide combines with the haemoglobin in the mother’s red blood cells to form a new compound, carboxyhaemoglobin, which cannot transport oxygen. Thus, it reduces the amount of oxygen reaching the fetus through the placenta.


Nicotine causes the arteries that bring blood to the placenta to narrow. Therefore, the amount of food substances reaching the fetus is also reduced. The mother’s health is also affected. She may suffer from lack of oxygen or chronic bronchitis


Evidence has shown that women who continue to smoke during pregnancy put their babies at risk in the following ways:


  • The brain development of the fetus is affected. The child may have learning difficulties in later life.
  • The fetus grows more slowly, so it is born smaller, fragile and may die within the first days of life.
  • There is a high risk of the baby being born prematurely.
  • There is a greater risk of baby being stillborn.