Medicines are preparations we take to prevent or treat disease and illness. Early medicines included herbs, roots, berries and a whole gamut of unusual substances, such as tooth of wolf, crocodile dung and gall of goat.
It is only in this century that pure drugs, chemical substances either synthesized or extracted from animal or plant tissue, have been used as medicines. The last 50 years have brought greater scientific knowledge to the development of drugs and an appreciation of their potentially dangerous side-effects. Paul Ehrlich (1854-1915), the German bacteriologist and pathologist, is regarded by many as the father of chemotherapy, the science of controlling disease by using specific chemicals. His most spectacular and valuable discovery was the drug salvarsan, also known as Ehrlich 606, which acts specifically against syphilis. Previously, the only treatment had been to feed the patient with poisonous derivatives of mercury, a method that only succeeded if the syphilis-causing organisms died before the patient did. The next major advance in the development of drugs came with Sir Alexander Fleming’s (1881-1955) famous chance discovery of penicillin, when he found that bacteria contained in a dish had dissolved in the presence of a stray mould. The substance that killed the bacteria was penicillin, but Fleming could not isolate the active material. The two Oxford scientists H.W. Florey (1898-1968) and E.B. Chain (1906-1979) succeeded in the early 1940s where Fleming had failed, and the penicillin they produced proved to be effective against many serious infections. Penicillin was the first of the class of drugs known as antibiotics, substances produced by living organisms such as bacteria and fungi that are capable of inhibiting the growth of other bacteria and viruses. The discovery of penicillin led to the discovery of a wide range of antibiotics that have since proved to be life-saving drugs.
Another group of drugs that have proved vital as medicines and were discovered in 1932 are the sul-phonamides. They prevent bacteria from multiplying and enable the body’s defence mechanisms to destroy them.
Until the discovery and use of antibiotics, sulphon-amides were used to treat bacterial infections. There are also drugs that correct abnormal body functions by their effect on the body’s cells and enzyme systems. These drugs include vitamins, hormones, and immunity-creating biological products, together with drugs that improve circulation and kidney function, suppress allergy and inflammation, reduce pain, and stimulate the nervous system.
Administration and absorption
The most convenient way to administer drugs is by mouth (orally). Tablets, syrups and capsules are easily taken by mouth. Although some are absorbed from the mouth mucosa or stomach, most are absorbed from the small intestine. This is because the inner lining of the intestine provides a huge surface area for absorption. The chemical structure of some drugs, however, prevents their being absorbed in this way and they have to be given by injection in order to reach the bloodstream. Some antibiotics and anaesthetics are examples of injected drugs. Some drugs, such as aspirin, may irritate the oesophagus and stomach and cause discomfort and vomiting. Other substances, such as insulin and vaccines, cannot be given orally because they are destroyed by the acidity of the gastric juices. One way of getting past these problems is to give the drugs a special coating (enteric coating) that prevents them dissolving until they reach the intestine, which has no acidity. This sort of coating is used in some aspirin preparations, but should not be confused with the simple gelatine coat of capsules.
Absorption of a drug is quicker on an empty stomach, and some drugs are recommended to be taken before a meal in order to achieve a high therapeutic level in the blood. But others, such as potassium chloride and propranolol, should be taken after a meal because their effects are needed over a longer period and slow absorption is desired.
Injections are usually given because the drug in question is not absorbed if taken by mouth, or because its action is needed quickly, or if very precise control of dosage is wanted.
The commonest routes for injections are subcutaneous (under the skin) or intramuscular (into muscle). The rate of absorption from these sites depends on the nature and concentration of the drug, and the local blood supply. A good supply near the injection site results in rapid absorption. Slow absorption, for example in psychiatric patients for whom long-acting drugs may be needed, can be achieved by dissolving the drug in oil.
When a patient is in a state of shock, blood flow to the skin and superficial muscles may be reduced to virtually nothing, and subcutaneous and intramuscular drugs will not be absorbed at all. The only reliable route in this instance is into a vein (’i.v.’, or intravenous injection). However, intravenous injections are not without dangers because tissues are exposed to very high drug concentrations. It is therefore advisable to inject drugs slowly.
Another method of administering a drug is by inhalation. The drug is contained in a measured-dose, pressurized aerosol. When the pressure is released, the dose is discharged as a fine spray which, if inhaled, is carried deep into the lungs, where the drug is absorbed almost as rapidly as if given by injection. When treating asthma and bronchitis, this method allows a patient to administer a rapid dose directly to the target area. Some drugs are given via the rectum if they are likely