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A drug is any substance taken into the body that modifies or affects chemical reactions in the body.

The liver is the site of breakdown of alcohol and other toxins

Medicinal Drugs


  • Antibiotics are substances that kill bacteria or prevent their growth, but do not harm other living cells
  • Most of them are made by fungi
  • It is thought that the fungi make antibiotics to kill bacteria living near them – bacteria and fungi are both decomposers, so they might compete for food
  • The first antibiotic to be discovered was penicillin. It is made by the fungus penicillium.
  • Penicillin kills bacteria by preventing the production of peptidoglycan that form the cell wall
  • Antibiotics do not work against viruses
  • Many antibiotics kill bacteria by damaging their cell walls
  • Viruses do not have cells walls, so they are unharmed by antibiotics

Development of 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
  • One type of bacteria that has developed resistance to a number of widely used antibiotics is called MRSA (methicillin-resistant Staphylococcus aureus)

How Development of Resistant Bacteria such as MRSA can be Minimized

  • Use antibiotics less often
  • Completion of antibiotic drug doses
  • Development of new antibiotics
  • Using combination of antibiotics
Misused Drugs

The Effects of Excessive Alcohol Consumption and Abuse of Heroin


  • Alcohol is a depressant and the consumption of large amounts of alcohol slow down the transmission of electrical impulses in the brain, so reactions are depressed, coordination is impaired and reasoned judgments become difficult
  • Poor judgments may lead to criminal activity and sexual promiscuity
  • Long-term excessive drinking can lead to addiction (alcoholism)
  • Heavy drinking also leads to liver damage such as cirrhosis. It can also cause brain damage, peptic ulcer in the stomach, and obesity
  • Drinking during pregnancy can damage the fetus, increase the risk of miscarriage or premature birth, and reduce the average birth weight


  • Heroin is a powerful depressant
  • It is a narcotic, producing a dream-like feeling of relaxation and reducing severe pain
  • It is very addictive, leading to dependency (addiction)
  • Withdrawal symptoms can be very unpleasant – involving cramps, sleeplessness, violent vomiting, sweating, and hallucinations
  • The body develops a tolerance to the drug, so an addict needs to take increasing amounts to achieve the same feeling
  • When injected using unsterilized and shared needles, there is a risk of infections such as hepatitis, HIV, etc
  • Addiction creates financial problems leading to family breakdown, criminal activity and sexual promiscuity

How Heroin Affects the Nervous System

  • Heroin produces its effects by interacting with receptor molecules at synapses
  • Synapses are tiny gaps between neurons, across which electrical impulses cannot jump
  • To maintain the transmission of the impulse, a chemical messenger called a neurotransmitter is released into the gap
  • When it reaches the neuron on the other side, receptor molecules are stimulated to generate and release new electrical impulses
  • Heroin mimics the transmitter substances in synapses in the brain, causing the stimulation of receptor molecules
  • This causes the release of dopamine (a neurotransmitter), which gives a short-lived ‘high’

Tobacco Smoking

  • Tobacco smoking can cause chronic obstructive pulmonary disease (COPD), lung cancer and coronary heart disease

The Effects of Tobacco Smoke on the Gas Exchange System and its Major Toxic Components

The Evidence for the Link Between Smoking and Lung Cancer

  • Although all forms of air pollution are likely to increase the chances of lung cancer, many scientific studies show, beyond all reasonable doubt, that the vast increase in lung cancer (4000% in the last century) is almost entirely due to cigarette smoking
  • There are at least 17 substances in tobacco smoke known to cause cancer in experimental animals, and it is now thought that 90% of lung cancer is caused by smoking
  • The fact that a higher risk of dying from lung cancer is correlated with heavy smoking does not actually prove that smoking is the cause of lung cancer
  • The alternative explanation is that people who becomes heavy smokers are, in some way, exposed to other potential causes of lung cancer, e.g. they live in areas of high air pollution or they have an inherited tendency of cancer of the lung
  • Theses alternatives are not very convincing, particularly when there is such an extensive list of ailments associated with smoking
  • This is not to say that smoking is the only cause of lung cancer or that everyone who smokes will eventually develop lung cancer
  • There are likely to be complex interactions between lifestyles, environments and genetic backgrounds which could lead, in some cases, to lung cancer
  • Smoking may be only a part, but a very important part, of these interactions

Performance-Enhancing Hormones

  • Some hormones belong to a class of chemicals called steroids
  • Steroid hormones include the reproductive hormones testosterone, estrogen and progesterone
  • Many steroid hormones stimulate metabolic reactions in body cells that build up large molecules from small ones
  • These reactions are called anabolic reactions. Steroid hormones that stimulate these reactions are called anabolic steroids
  • One type of reaction that is stimulated by anabolic steroids is the synthesis of proteins from amino acids
  • Testosterone causes more proteins to be made in muscles, so the muscles become larger and stronger
  • Athletes and others have taken anabolic steroids to increase their muscle and strength
  • These hormones can help athletes to train harder and for longer periods of time