Medication Action: Pharmacodynamics

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Medication Action: Pharmacodynamics is the action of the medication on the body. In our last article, we remember that Pharmacokinetics is the action of the body on a particular drug.

Upon administration, a medication travels to the target cells to produce its desired effect. This results in an alteration at the cellular level. Pharmacodynamics is the process in which medications alter the cells of the body.

Medication Actions: Receptor Sites

Each medication is different in the way it acts and the types of action it produces. One mechanism medications use to interact with the cells is receptor sites. Receptor sites are specialized structures on the cells. The medication and the receptor site fit like a lock and key.

Once a medication joins with a receptor site, it can initiate a number of cellular responses or actions. This depends on the cell and the medication. This interaction can activate, deactivate, increase, decrease, stimulate or inhibit cellular responses of the body.

Types of Medication Action

Therapeutic Effects

When a medication is administered, it has an expected or desired physiological response. This effect is the therapeutic effect. The therapeutic effect is usually predictable.

Each medication has its own desired effect or therapeutic effect. When you know what the therapeutic effect should be, you are better able to determine the patient’s response to the medication.

Adverse Effects/Side Effects

An adverse effect is when a patient does not have an expected therapeutic effect and instead has an undesirable effect. Adverse effects are not intended and usually not predictable.

The response to the medication can be severe and can cause harm to a patient. When a medication causes an adverse effect, the medication is usually discontinued.

However, some adverse effects are predictable and tolerated. When the benefit of a medication outweighs the adverse effect, the adverse effect is tolerated.  The adverse effect is normally treated with another medication.

When a patient has a predictable effect that cannot be avoided, this is a side effect. A side effect can be dangerous or harmless. This all depends on the patient and the medication.

As with adverse effects the benefit may outweigh the side effects. And also, as with the adverse effects, the medication will be discontinued if the side effect outweighs the benefit.

That is why it is important for the nurse to understand the patient’s condition and the desired or intended effect of the medication therapy. Regardless, it is always important for the nurse to monitor for any adverse effects of medications. Some adverse effects and side effects are more severe than others.

Medication Action: Pharmacodynamics

Toxic Effects

A toxic effect occurs when there is an accumulation of medication in the blood. The body has an inability to metabolize or excrete the medication before the next dose of medication. The medication then accumulates in the body.

If an excess amount of medication accumulates in the body, there can be severe consequences. The toxicity of the medications can cause organ damage, system damage, or even death.

This happens when one of the organs that metabolize or excrete the medication is slowed or impaired. Therefore the medication is not metabolized or eliminated. And, the patient eventually becomes toxic. The nurse should always monitor for toxic effects of medications especially in the elderly.

Idiosyncratic Effects

When a patient has a reaction to a medication that is not desired, predicted, or expected, this is an idiosyncratic effect. In this situation, the patient will overreact, or underreact to the medication.

Some patients will have a totally different reaction. These reactions are unusual and completely opposite of the response that is expected. When a patient has an idiosyncratic effect, it tends to be unique to that patient.

Again, monitor your patient and listen to patient complaints. Be aware that idiosyncratic effects occur.

Allergic Reactions

Another unpredictable response to medications is an allergic reaction. An allergic reaction may also occur hours to days after an initial dose.

The patient may develop the allergic response with the first dose or with repeated doses. The medication, its preservatives, or a metabolite may be what causes the allergic response.

An allergic reaction varies from one patient to another. The reactions can range from a minor reaction to a more serious reaction. With continued administration, the symptoms of the reaction may become more severe.

An anaphylactic reaction is the most serious of the allergic reactions. This anaphylactic reaction or anaphylaxis is a life-threatening condition that can cause respiratory distress due to constriction of the bronchiolar muscles. This eventually culminates in cardiovascular collapse. Anaphylaxis can occur suddenly and without warning.

The nurse should always be aware of the patient’s history especially monitoring for any previous allergic reactions to medication or medication within the same family.

Drug tolerance

Drug tolerance occurs when medication no longer has the desired effect because the patient’s body has become accustomed to the medication. In this situation, a larger dose of the medication must be given to produce the desired therapeutic effect.

Medication Interaction

When a patient is taking more than one medication, one medication action may increase, decrease or modify the action of the other medication. This is a medication interaction.

Medications may affect the absorption of another medication, especially when given together. Also, one medication may also alter the metabolism of another medication. In some situations, metabolism may be induced or it may be inhibited. Some medications can even affect the elimination of another medication.

Medication interactions can be dangerous. Always monitor the patient for the desired therapeutic effect and medication interactions, especially if a patient is on multiple medications. And, nurses should be familiar with common medication interactions.

Synergistic Effect

A synergistic effect happens when two medications with different mechanisms of action produce a greater effect than if they were given separately.

This type of effect is sometimes desired. A healthcare provider may prescribe medications for their combined reaction in order to cause a beneficial effect for the patient.


Medication Action: Pharmacodynamics is the action of the medication on the body or what the medication does to the body. Medication administration requires monitoring the patient for responses and reactions.

Also, it is helpful to remember the rights, the routes, the pharmacokinetics, and the pharmacodynamics of medication administration for patient safety and to decrease medication errors.


Taylor, PhD MSN, RN, Carol, et al. Fundamentals of Nursing: The Art and Science of Person-Centered Nursing Care. 8th ed., Philadelphia, Wolters Kluwer Health/Lippincott Williams & Wilkins, 2015.

Potter RN, MSN, PhD, FAAN, Patricia A., and Anne G. Perry RN, EdD, FAAN. Fundamentals of Nursing. 9th ed., St. Louis, Mosby Elsevier, 2017.

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