Potency Vs. Efficacy

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The terms "potency" and "efficacy" are often used interchangeably in discussing medications. However, they are two distinct terms that speak to different aspects of a medicine. A potent drug is not always the most efficacious. In contrast, many drugs with high efficacy have a low potency.

Definitions

  • Potency is the relationship between the the dose of a drug and the therapeutic effect. It refers to the drug's strength. A drug is considered potent when a small amount of the drug achieves the intended effect. Efficacy is the ability of a drug to produce the desired therapeutic effect. Efficacy means that the drug is effective. When comparing two drugs that work equally, the one with the lower dose has a higher potency. They have equal efficacy.

Significance

  • A medication must have demonstrated efficacy to gain approval from the U.S. Food and Drug Administration. All drugs that the FDA approves have established levels of efficacy. Potency is significant with regards to side effects. Smaller, less potent, doses of medications tend to cause fewer undesirable effects while achieving the intended therapeutic effect.

Considerations

  • When choosing a drug therapy, doctors must consider both the efficacy and potency of a drug. Though a drug may be more effective in its treatment, it may cause more side effects, have greater toxicity or cost more. The choice of a more potent or less effective drug depends on the disease status, the patient's drug tolerance and the doctor's practice experience.

Misconceptions

  • It is commonly thought that more potent drugs are more effective. Because it takes less of a drug to achieve an effect does not mean that the drug is more effective. For example, both 500 mg of acetaminophen and 200 mg of ibuprofen resolve a headache. Although ibuprofen is more potent because it requires a lower amount of drug, both drugs eliminate the headache. They are equally effective.

Examples

  • Highly potent drug classes include chemotherapeutic (cancer) drugs, antihypertensive (blood pressure) drugs and anitlipid (cholesterol) drugs.

    High efficacy drug classes are analgesic (pain) medications, antibiotics, antihistimines and decongestants.

References

  • Photo Credit "ibuprofin" is Copyrighted by Flickr user: Muffet (liz west) under the Creative Commons Attribution license.
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