To understand pharmacology (and not just memorize hundreds of drugs), it’s helpful to have a basic understanding of the concept of pharmacodynamics. Pharmacodynamics refers to what the drug actually does in the body. This is the drug’s mechanism of action and intended effect.

A drug exerts its effect by interacting with its target tissue or cell to produce the desired result. Many drugs do this by interacting with receptors. We’ll dive into the different receptors in an upcoming lesson. For now, just understand that a receptor is any cellular component the drug binds to and can be located on the surface of the cell or within the cell itself. Drugs can also exert their influence by affecting cell function itself.

When a drug interacts with a receptor, it can enhance the action of the cell or suppress the action of the cell. We call the enhancers agonists, and we call the suppressors antagonists or blockers. A prime example are “beta blockers” which slow the heart rate.

Two other key pillars of pharmacodynamics are efficacy and potency. Efficacy refers to the ability of a drug to produce the desired result. Diuretics are an excellent example of this. Furosemide is a diuretic that causes the individual to eliminate sodium and water through the renal system to lower blood pressure and improve fluid balance. When you compare furosemide to another diuretic, hydrochlorothiazide, furosemide causes the individual to eliminate more sodium and water than hydrochlorothiazide does. So, furosemide has greater efficacy than hydrochlorothiazide.

Potency refers to how much drug is needed to produce the desired result. Let’s say you are comparing two drugs: Drug A produces the desired result after 10 mg are given, and Drug B produces the desired result after 25 mg are given, Drug A is more potent. A perfect example of this is hydromorphone vs morphine. A 1.5 mg dose of hydromorphone is effectively equal to a 10 mg dose of morphine because hydromorphone is much more potent.

Pharmacodynamics can be affected due to other medications the patient is taking, genetic mutations, some disease conditions including malnutrition, and the aging process. This is why you’ll often hear the phrase “start low and go slow” when it comes to geriatric medication administration. 

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