The principles of pharmacology

Pharmacology basics

Pharmacology is the study of the properties, actions, and uses of drugs. A drug is a chemical substance used to cure, treat, prevent, or diagnose disease. In the ambulatory care setting, medical assistants deal with medication, from history taking to administering medications. Medical assistants must have a general understanding of the classification of drugs. They need to know how to pronounce the medication names. They must know how to give medications, the dosage to give, and typical side effects. New medications are continually being developed and released for patient treatment. Thus, medical assistants must stay updated on medications.

Medications have been around for a long time. The first medications came from natural products found in our ancestors’ living environment. Today, with advancing technology, most medications are created in a laboratory setting. These sources and others will be explained, along with the uses of medications.

In addition, this section presents a basic description of how medication enters, moves through, and exits the body. This knowledge will help the medical assistant identify patients who may be at risk for medication issues because of their age or a disease process.

Sources of drugs

Drugs are either created from natural sources or made synthetically in a laboratory. Plants, animals, minerals, and microbiologic sources are natural sources of drugs.

Natural sources of drugs

Plants are the oldest source of drugs. Our ancestors found that different plants helped with different symptoms. Leaves, bark, stems, roots, and fruits have been used in medicinal preparations through the years. Some examples of medicinal plant sources include the following:

• Digitalis, an antiarrhythmic medication, comes from the purple foxglove flower.
• Nicotine comes from tobacco leaves.
• Quinidine, an antimalarial medication, comes from the bark of the cinchona tree.

Animals are also a source of medications. Natural substances are extracted from animal tissues and organs. Heparin, an anticoagulant, comes from pig intestines. Lanolin is found in topical preparations used to protect the skin. It comes from the sebaceous glands of sheep. Other medications are developed with lactose and gelatin, which are from animals.

Minerals and microbiologic substances are other natural sources of medications. Examples of mineral use include the following:

• Iron is used to treat iron-deficiency anemia.
• Iodine is an antiseptic.
• Zinc is used as a supplement and is found in topical pastes for wounds.

One microbiologic source is Penicillium chrysogenum. This is a fungus that creates penicillin.

Synthetic sources of drugs

Many medications originated in nature but have been recreated in the laboratory setting. For instance, insulin initially came from cattle and pigs. Many people developed allergies to the insulin. Now synthetic insulin is widely used. Biotechnology and genetic engineering techniques are continually being used to create new medications. With the help of technological advances, individualized medications are also being produced. (Individualized medications are discussed later in the chapter.) Synthetic medications are cheaper to produce because they are created in mass volumes. The quality of synthetic medications can also be controlled.

Uses of drugs

When you are studying medications, it is important to identify the uses of drugs. Why are they being prescribed? What do they do in the body? Some medications may have more than one use. There are eight common uses of drugs:

• Prevention: Drugs used to prevent diseases. For example, vaccines are given, and the body creates antibodies to protect against specific diseases.
• Treatment: Drugs that relieve the symptoms while the body fights off the disease. For example, acetaminophen, an antipyretic medication, brings down a fever while the body fights off a viral infection.
• Diagnosis: Drugs used to diagnose or monitor a condition. For example, a contrast medium (radiopaque dye) is given to highlight organs on x-rays.
• Cure: Drugs that eliminate the disease. For example, amoxicillin, an antibiotic, is used to cure strep throat.
• Contraceptive: Drugs used to prevent pregnancy. For example, Depo-Provera is an injectable contraceptive medication.
• Health maintenance: Medications used to maintain or enhance health. Vitamins and minerals are examples.
• Palliation: Drugs that do not cure or treat the disease but improve the quality of life. For example, morphine, an analgesic, is commonly used by patients with cancer.
• Pharm Replacement: Drugs used to increase the blood levels of naturally occurring substances in the body. For example, levothyroxine is used for patients with hypothyroidism.

Pharmacokinetics

Pharmacokinetics is the study of drug absorption, distribution, metabolism, and excretion in the body. Through pharmacokinetics, we understand when a medication starts to work in the body. We know how it moves through the body and what organs metabolize and excrete the drug from the body. Some of the patients you will be working with will have greater risks of side effects and toxicity. Understanding the basics of pharmacokinetics will help you identify those at greater risk for problems.

Absorption

Drugs can be administered in many ways. Route is the means by which a drug enters the body. Where a drug enters the body is considered the site of administration. Absorption is the movement of a drug from the site of administration to the bloodstream. The following are commonly used routes:

• Oral (po, PO): Taken by mouth
• Sublingual (SL): Placed under the tongue to dissolve
• Buccal: Placed between the cheek and the gums to dissolve and absorb quickly
• Intramuscular (IM): Injected into the muscle
• Subcutaneous (SUBQ): Injected just below the skin; moves into the capillaries or the lymphatic vessels and is brought to the bloodstream
• Intravenous (IV): Injected directly into the bloodstream

Factors influencing absorption

The rate of absorption is influenced by the following factors:

• Route: Oral medications need to pass through the gastrointestinal (GI) tract. This takes time. IV medications are directly administered into the bloodstream and have virtually no absorption time. They start working faster than drugs given by other routes.
• Blood flow to the absorption area: The greater the number of blood vessels, the quicker the absorption. Medication given by the sublingual and buccal routes is absorbed quickly into the bloodstream. These sites are rich in blood vessels. IM medications absorb quicker than SUBQ medications. The muscle tissue has more blood vessels than the subcutaneous tissue.
• Ability of the medication to be absorbed: Liquid medications are easier to absorb than solid medications. Solid medications need to be broken down before they absorb. Acidic medications are absorbed in the stomach. Base (or alkaline) medications are absorbed in the intestines.
• Conditions at the site of the absorption: Some medications must be taken with food, which can slow the absorption of the medication. Typically, medications taken on an empty stomach can be absorbed faster. The intestines provide more surface area than the stomach for the absorption of medications.

Distribution

Once the drug is absorbed into the blood, it rapidly circulates through the body (unless it must go through the liver). During this time, the drug is brought to the body tissues. The movement of absorbed drugs from the blood to the body tissues is called distribution. The speed of the drug’s movement from the blood to the tissues varies greatly. Some drugs bind with proteins in the blood and move slowly into tissues. Some drugs accumulate in certain tissues. These tissues act as reservoirs, slowly releasing the drug into the bloodstream and keeping the blood levels from decreasing too rapidly. This process prolongs the effect of the drug. Circulation issues (e.g., peripheral artery disease) can also slow the distribution of medication.

For the medication to move into certain organs, it must be able to pass through the tissues. The blood-brain barrier allows only certain fat-soluble medications to pass into the cerebrospinal fluid and the brain. In comparison, the placental membrane allows most drugs to pass through from the mother to the baby in utero. Therefore, only certain medications are prescribed during pregnancy.

Routes affect the dose

Oral medications are absorbed in the stomach or the intestines. The blood containing the absorbed digestive nutrients and drugs passes through the hepatic portal vein and the liver before it circulates to the rest of the body. In the liver, some of the drugs are chemically altered. Some of the active drugs are lost during this first pass through the liver. This reduces the amount of drugs in the circulating blood that can be used.

For instance, a person is having an allergic reaction and needs Benadryl. If the drug is to be taken orally, the person takes 50 mg. If it is to be given intravenously, 10 mg may be given. This is because all 10 mg gets into the bloodstream, whereas some of the 50 mg is lost as it passes through the liver before circulating through the body. It is important for a medical assistant to realize that doses (e.g., 10 mg) may vary based on the route used to give the medication.