What Is the Difference Between Mcg and Mg Measurement?

Receiving a correct dosage is one of the five rights of medication administration, for both prescription and over-the-counter drugs. Two examples of units of measurement common in medicine are mcg and mg. Though they're very similar, they're two different units of measure, and not knowing the difference can result in serious consequences if they're not read correctly.

Mcg Vs. Mg Measurement (Image: d3sign/Moment/GettyImages)

Definition of a Mcg

The metric system uses the abbreviation mcg to represent micrograms. A microgram is equivalent to a millionth of a gram or a thousandth of a milligram. In the International System of Units measurements, the mcg is one of the smallest frequently used units. To convert milligrams to micrograms, multiply 1,000 by the mass. For instance, if the mass is 1.25 mg, multiply by 1,000 to get 1,250 mcg.

Definition of a Mg

A milligram (mg) is a metric system unit that represents a thousandth of a gram. Because of its larger size, a mg is a more commonly used measurement than a mcg. One thousandth of a mg is one mcg, and 1,000 mcg equal one mg. To determine how many mg there are in a number of grams, multiply by 1,000. For example, five grams equals 5,000 mg.

Examples of Mcg

The potency of the generic drug fentanyl is the reason the drug is measured in micrograms. An example of a proper fentanyl transdermal patch is 25 mcg/hour. The vasodilator nitroglycerin is another form of medication measured in micrograms; an example of a dosage is 100 mcg. Use of the microgram is common in microbiology tests that require the use of antibiotics, including penicillin, oxacillin, erythromycin and the broad-spectrum chloramphenicol.

Examples of Mg

Milligram are the more commonly used measurement for non-liquid medications. A doze of benzodiazepine alprazalom, more commonly known as Xanax, is measured in milligram increments. Typically, less than a milligram is administered in single dosage. Another example is the angina medication Ranexa, also known as ranolazine; a single dose as high as 1,000 mg is often prescribed. Nutrition labels are another place where milligrams are used, describing sodium or cholesterol content in a food product.

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