Principles of Drug Action Medical Math Chapter 2 Buffy Ryan, RN

Drug Action The chemical in a drug combines with or alters the molecules in body cells so that it changes the way the cells work. The 4 main drug actions are: 1. Depressing 2. Stimulating 3. Destroying cells 4. Replacing substances

Drug Action Drugs usually speed up or slow down the ordinary processes the cells carry out Example: antihistamines slow the body’s natural reactions to irritation, and stimulants speed up the energy-producing functions of cells

Drug Action Some drugs destroy certain cells or parts of cells Example: antibiotics kill disease microorganisms; 5FU and methotrexate kill cancer cells

Drug Action Some drugs action is to replace or supplement natural substances that the body lacks due to organ malfunction or poor nutrition Example: Potassium chloride and calcium carbonate

4 Basic Body Processes That Affect Drug Action Absorption Distribution Metabolism/Biotransformation Excretion

Absorption Absorption is the passage of a drug from the site of administration into the bloodstream How quickly a drug is absorbed is important because it determines how soon it will start it’s action The route of administration can affect the rate of absorption

Absorption Route of Administration Sublingual(under tongue) Oral(by mouth) Subcutaneous Intramuscular Intravenous Absorption begins In the mouth In the stomach/intestine Under the skin In the muscle In the blood stream

Distribution Distribution is the transport of the drug from the bloodstream to the body tissues and intended site of action Some of the drug passes through the capillaries thin walls (alcohol passes easily thru capillaries and into tissues quickly) Distribution time varies with different drugs

Metabolism/Biotransformation This is a series of chemical reactions that inactivate a drug by converting it into a water-soluable compound so it can be excreted by the body Sort of like digesting food, the body uses enzymes to break down the drug and detoxify toxic substances The liver is where most metabolism takes place, some occurs in the lungs, intestines, kidneys, & blood

Excretion Excretion is the body’s way of removing the waste products. Most drugs leave the body through the kidneys and large intestine The lungs, sweat, hair, and milk glands also excrete An elderly person with kidney disease has an increased risk of toxicity….WHY?

Patient Education Concerning Excretion Increase fluid intake to aid in excretion Cough & deep breathe after anesthesia Chew gum or hard candy to decrease Avoid laxatives, they speed up drug excretion Keep skin clean to avoid irritation from excretion(sweat) Unpleasant taste from excretion via saliva Improper diet & lack of activity slow excretion Pregnant or nursing moms should check with MD

Factors Affecting Drug Action

AGE Infants body systems are immature The body systems of the elderly may have decreases in kidney and liver function Smaller doses may be required to treat the young and old

SIZE The average adult dose is calculated for a person age weighing 150lbs As a result, doses for the young, elderly, or obese must be calculated by body weight

DIET Combining certain foods with certain drugs can alter the effects EX: Tetracycline/Milk Coumadin/Vitamin K

SEX Women are generally smaller than men and have more body fat Pregnant women must always consult their physician because of a drugs risk to the fetus

GENETIC FACTORS Heredity can affect a persons metabolism and excretion process Some people lack the naturally occurring enzymes to break down drugs for excretion

PATHOLOGICAL CONDITIONS Diseases can strongly affect how patients respond to drugs Heart disease, kidney failure, diabetes, and low blood pressure can affect drug action Cancer can also affect drug action, requiring a stronger dose

PSYCHOLOGICAL FACTORS A positive attitude is likely to respond well to medication Some patients have taken a placebo and reported feeling better Strong feelings such as worry, anger, fear or jealousy can affect drug action

ROUTE OF ADMINISTRATION A drug acts more quickly when injected directly into the bloodstream Medications taken by mouth take the longest time to show effects

TIME OF ADMINISTRATION Important to give meds at time they were ordered Some drugs need to be taken with a meal, others are absorbed more quickly on an empty stomach

DRUG-TAKING HISTORY A patient can build up a tolerance to a particular drug Some drugs can interact with each other Always check allergies

ENVIRONMENTAL CONDITIONS Heat and cold can affect drug action Heat relaxes the blood vessels, speeds up circulation, drugs act faster Cold slows action by constricting vessels

DRUG EFFECTS

Drug Effects Therapeutic effect – what you want Side effect – what you may get along with the therapeutic effect (some good, some bad) Ex: Morphine – therapeutic effect – kills pain, side effect – respiratory depression/constipation/urine retention

Classification of Drug Effects Local – affects mainly the area it enters or is applied. Benadryl ointment applied to a bug bite on skin. Systemic – travel throughout bloodstream to affect cells or tissues. Benadryl capsule taken by mouth due to allergic rash.

Adverse Reactions What you don’t want!!! Idiosyncrasy – opposite of expected effect Tolerance – lessened effect Cumulation – stronger effect Toxicity – different symptoms affecting organs Synergism – stronger effect when more than one drug taken

Adverse Reactions Antagonism – weaker effect when more than one drug taken (tetracycline & antacid) Potentiation – effect of one drug increases effect of another drug (tylenol & codeine) Interaction – therapeutic or adverse effect on body Physical dependence – physical need for a drug Psychological dependence – mental craving for drug