Drug Dose vs. Drug Dosage

Although people often use dose and dosage interchangeably, these terms have very different meanings. By adding the suffix -age to the base dose, we are implying an regulated amount, a differentiation similar to the words mile and mileage.

Definition 

According to the American Medical Association (AMA) Manual of Style, the definitive guide on medical writing and style, dose and dosage each have very specific meanings.

A dose refers to a specified amount of medication taken at one time. By contrast, dosage is the prescribed administration of a specific amount, number, and frequency of doses over a specific period of time.

According to the AMA, drug doses are expressed in conventional metric mass units (for example, milligrams or milligrams per kilogram) rather than in molar SI units. Moreover, certain drugs (such as insulin or heparin) may be prepared as mixtures and have no specific molecular weight, thereby precluding their expression in mass units.

Although other drug dose units such as drops (for ophthalmologic preparations), grains (for aspirin), and various apothecary system measurements (eg, teaspoonfuls, ounces, and drams) may be encountered clinically, these units generally are not used.

In other words, a dose is simply an amount (think weight) of a medication which is administered at one specific time. Whereas, a dosage is the dose, or amount of drug, attached to a time frequency. A dosage guides a drug regimen.

Using Tylenol As an Example

Different types of Tylenol (acetaminophen) come in different doses. Choice of which specific type, or dose, of Tylenol, depends on the condition being treated. Dosages also vary by the condition being treated.

Here are some different dosages of Tylenol dosages:

  • For minor aches and pains, take 2 tablets of Regular Strength Tylenol (dose 325 mg each tablet) every 4 to 6 hours as needed (q4 to q6 PRN). Do not exceed more than 10 tablets in 24 hours, and if the pain persists for more than a day or two, see a physician.
  • For more severe aches and pains (like a bad headache), take 2 Extra Strength Tylenol caplets (dose 600 mg each caplet) every 6 hours as needed (q6 PRN). Do not exceed more than 6 caplets a day. If pain persists for more than a day or two, see a physician.
  • For arthritis pain, take 2 Tylenol Arthritis Pain caplets (dose 650 mg each caplet) every 8 hours as needed (q8 PRN). Do not exceed more than 6 caplets a day.

Other Examples

Glucophage (metformin) is an oral prescription medication commonly used to treat type 2 diabetes. A common dosage of Glucophage for people newly diagnosed with type 2 diabetes is one 500 mg. (the dose) twice a day (the interval).

Each tablet of Tylenol PM contains 500 mg. (the dose) of acetaminophen, for pain relief, and 25 mg. (the dose) of diphenhydramine, a nighttime sleep aid. The drug facts label of this over-the-counter medication recommends that adults and children over age 12 years take a dosage 2 tablets at bedtime (the interval).

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Article Sources
  • AMA Manual of Style, Oxford University Press, 10th edition, 2007.