Medicines are given in several different ways. In all cases where practical, medication will be given by mouth, but in many cases it is necessary to use other routes to get the drug into the body in sufficient quantities. Certain drugs are not absorbed adequately from the intestinal tract; others are changed chemically by the digestive juices, and still others are irritating to the stomach lining. So, with many drugs it is not possible to use the oral routes at all. Certain drugs will work only if put directly into the blood stream and others are most efficient injected into a muscle or the fatty layer beneath the skin.
In all cases, the drug must eventually reach the blood stream before it can display its action. Drugs taken orally and those injected into muscle or fat must be absorbed by osmosis into the blood. The blood stream delivers the drug to all parts of the body, and it will then perform its action on the organs it affects. Many drugs can be given by any route. Those given into the veins will begin work immediately, while those given by other routes are delayed until they are absorbed into the blood stream. The time for osmotic absorption varies with the drug, but in general it requires about 20 minutes for absorption of drugs given by hypodermic (“hypo”) (under the skin into muscle or fat) injection, and slightly longer when given by mouth. In emergencies where immediate action is desired, the intravenous route will be used.
Naturally, any substance which is to be injected into the vein or under the skin must be absolutely sterile, so as not to introduce germs into the body. Certain compounds are so chemically unstable that they cannot be sterilized, and these of course can be given only by the oral route. Some drugs are too irritating to the vein lining and subcutaneous tissues, so they also are limited to the oral route. There are factors about each medication which dictate the route of administration. In addition to the common routes of administering drugs by injection and by mouth, there are also other methods of getting drugs into the body. Diseases of the brain and the spinal cord can be treated by placing drugs into the spinal fluid via lumbar puncture. Other drugs may be inhaled for treatment of diseases of the lungs or for absorption through the lungs into the blood stream. Medication can also be absorbed from surface membranes by placing them in the mouth, nose, vagina, rectum, or even the eye. Many drugs are used for their local action without depending on their absorption and transport by the blood stream. These drugs and ointments are placed directly on the area where their effect is needed. This type of drug treatment is used for diseases of the skin, eye, ears, nose, mouth and throat, rectum, bladder, and vagina.
The dosage of drugs administered by any route is dependent on a number of factors. Your doctor carefully calculates the dosage of each medication he orders for you according to your age, body weight, physical condition, and other factors. As a rule, the action of drugs is more profound in children and aged individuals, and obviously when a drug is distributed throughout the body of a large individual more is required to reach the same effect as in a smaller person. In weakened physical conditions, pregnancy, shock, and anemia, dosages of certain drugs must be modified. Other factors which may influence the dosage of certain drugs are sex, route of administration, time of administration, and other drugs being given concurrently. Certain agents may act differently in males than females because of the different hormones present in their systems. With most drugs the oral route requires a slightly larger dosage than an injection route, because absorption may not be so complete. Absorption efficiency from the intestinal tract is also influenced by the ingestion of food, so that medications taken at mealtime often require higher dosages. Certain drugs when used in combinations have an effect on each other, so occasionally dosages have to be modified to acquire the desired effects. The frequency with which a drug must be given is determined by the rate of its excretion from the body. Drugs which stay in the blood stream for 4 hours will have to be repeated every 4 hours to maintain a continuous effect. Each drug has a specific duration of effect. Some drugs will be ordered in a frequency dictated by their produced effect, such as those used to control pain after operations which may be given each time needed rather than continuously.
In many situations, certain drugs are contraindicated altogether. In cases where one drug counteracts another, the physician must choose the one whose effect is needed the most. The physical condition of the patient may preclude the use of certain drugs. For example, in pregnancy, drugs affecting the womb are contraindicated; in cases of hemorrhage, drugs which delay clotting must be avoided; and in shock and other depressed states, sedation agents can be harmful.
This science of drugs and their uses is called pharmacology. It is a very complex subject, since there are so many facts to be known about so many agents. These principles, of course, apply not only to your hospital treatment, but also to all phases of your doctor’s care for you. Pharmacology is an exact science, and since there are so many factors your physician takes into consideration each time he prescribes, you can realize the importance of following directions specifically. It is also apparent why a prescription for one illness must not be used for a subsequent illness, even though the symptoms may appear to you to be the same. The art of compounding drugs according to the physician’s prescription is called pharmacy.