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Home Care Association of New Hampshire

Licensed Nursing Assistant Medication-Certified

Drug Action and Interactions

Drug Action

Drug action is the physiochemical interaction between the drug molecule and molecules in the body that alters a physiological process of the body in one of three ways

  • Replacement: a drug replaces what the body once made
  • Interruption: the drug interferes with a physiological process, for example, antihypertensive drugs interfere with the process that constricts blood vessels and that may cause blood pressure to rise
  • Potentiation: a drug that stimulates a physiological process; for example, Lasix stimulates the kidneys to excrete urine

The pharmaceutic phase is the form of the drug:

  • Tablet
  • Capsule
  • Liquid
  • Elixirs
  • Syrups

Drugs contain active and inactive ingredients. The active ingredient is the drug that causes the desired pharmaceutical response. The inactive ingredient, also called excipient, is what helps deliver the drug.

There are fillers and inert substances that give the drug their size and shape. The coating around tiny particles of a capsule that causes a timed release action of the drug is an inactive ingredient.

Most drugs (80%) are administered orally (po). They are carried to the small intestine where they are absorbed into the bloodstream. Absorption in children and the elderly is affected by the pH in their gastrointestinal tract and is slower that in an adult.

Some drugs are more effective if absorbed in the small intestine rather than the stomach. Enteric coated medications resist disintegration in the stomach so they will be absorbed in the small intestine. Never crush an enteric pill, it would then be released in the stomach. This could damage the stomach lining.

Absorption begins where the drug is administered. How long the drug will be effective and how much drug is needed depends of the route of administration, the dose of the drug and the dosage form. The blood flow to the site of administration affects the rate of absorption.

Generally 20% to 40% of drugs given by mouth (po) reach the blood stream. Only when a drug is administered intravenously does 100% of the drug reaches the blood stream. Factors that affect the amount of drug reaching the blood stream include:

  • Form: tablet, capsules, slow-release, liquid, transdermal patch, suppository and inhalation
  • Route: po, topical, parenteral and rectal
  • GI: the ability of muscosa in the GI tract impacts the ability to absorb drug particles and ability to move food through the digestive tract
  • Food: some drugs are absorbed better it taken with certain foods, while some foods block or slow absorption
  • Drugs: some increase or decrease another drug’s absorption when both drugs are taken together
  • Liver metabolism: liver disease or malfunction can prevent or delay the metabolism of a drug
  • Concentration: a higher portion of active ingredient in a dose increases the amount of drug particles that are absorbed
  • Cell membrane: single layer membrane, such as the small intestine, increases absorption, while drugs are more slowly absorbed through multiple layers such as the skin
  • Surface area: a larger surface area, such as in the small intestines, absorbs drugs faster than the smaller area of the stomach

Drug Elimination

Drugs accumulate in a reservoir and are gradually absorbed and eliminated from the body. This metabolism is called biotransformation and occurs in the liver. Elimination occurs mainly through the kidneys, although some drugs are excreted in bile, feces, lungs, sweat and breast milk. Patients who have liver damage or disorders would be prone to drug toxicity.

Categories of Drug Action

A drug is categorized by the type of action it causes on the body. There are four actions:

  • Stimulation or depression: these either increase of decrease cellular activity
  • Replacement: these drugs replace an essential body compound like estrogen or insulin
  • Inhibition: these interfere with the bacterial cell and limit bacterial growth or eliminate bacteria
  • Irritation: these drugs irritate cells to cause a natural response that has a therapeutic effect such as a laxative

Side effects

A side effect is a physiologic effect other than the desired effect. Some are desirable, some are not. They can be predictable and some are related to the dose. Drug effects can also be related to other factors including:

  • Gender- women generally have smaller livers and less fat
  • Environment-cold, heat, sensory deprivation or overload and hypoxia
  • Time of administration-might be influenced by the presence or absence of food
  • Pathologic state- can react differently if the patient is having pain, anxiety, circulatory distress or hepatic/renal dysfunction
  • Idiosyncrasy- an abnormal response that is unpredictable and/or unexplainable
  • Tolerance- the patient has a decreased physiologic response after repeated administration of the drug
  • Drug dependence- either a physical or psychological dependency
  • Drug interaction- administering one drug increases or decreases the response of a previously given drug
  • Synergism- a better pharmaceutical response is achieved through the interaction of two drugs that are given
  • Potentiation- concurrent administration of two drugs increases the response of one of those drugs
  • Toxic effect- the drug exceeds the therapeutic range due to an overdose or by accumulating in the body
  • Tachyphlaxix-this is drug tolerance and occurs with narcotics
  • Placebo effect-the patient receives a psychological benefit
  • Pharmacogenetic effect- the effect is unexpected due to genetic factors
  • Allergic response

Disclaimer: Adapted to home care by the Home Care Association of NH from the NH Healthcare Association’s MNA program curriculum.