milligram
Meaning of milligram: One thousandth of a gram, the unit most drug doses are written in and the one most often confused with a microgram.
Definition of milligram
A milligram is a unit of mass equal to one thousandth of a gram, written as mg. It is the unit most medication doses are expressed in, and it also appears in laboratory results and on nutrition labels. It is a small quantity: a thousand milligrams together weigh about as much as a paper clip, and there are roughly twenty-eight thousand of them in an ounce.
How does it compare with other units?
| Unit | Symbol | Relation to a gram |
|---|---|---|
| Kilogram | kg | One thousand grams |
| Gram | g | The base unit |
| Milligram | mg | One thousandth of a gram |
| Microgram | mcg | One millionth of a gram |
| Nanogram | ng | One billionth of a gram |
In the older system an ounce is about twenty-eight grams and a pound about four hundred and fifty grams, which is why doses converted between systems produce awkward numbers.
Why is the milligram and microgram distinction dangerous?
They differ by a factor of a thousand, and several important drugs are dosed in micrograms while others are dosed in milligrams. A dose written with the wrong unit is therefore a thousandfold error rather than a small one. The Greek letter used in the proper symbol for microgram is easily misread as an m in handwriting, which is why prescribing systems and hospital policies commonly require the word microgram to be spelled out rather than abbreviated. Similar reasoning lies behind rules against a trailing zero after a decimal point, which can be read as a tenfold increase if the point is missed.
Where else does it appear?
Laboratory results often use milligrams per deciliter for substances in blood. Drug doses for children and for many cancer treatments are given as milligrams per kilogram of body weight, so the dose is calculated rather than fixed. Nutrition labels use milligrams for sodium, calcium and most minerals. Care is needed with two other units that look similar but measure something different: millimoles count particles rather than weight, and milliequivalents count electrical charge, so converting between them and milligrams requires knowing the substance involved.
Why is the metric system used at all?
Because every step is a power of ten, so conversion is a shift of the decimal point rather than an arithmetic problem. That property is what makes weight-based dosing practical, and it is why medicine uses grams and liters even in countries where daily life uses pounds and pints. The system also fixes a single meaning for each prefix, so milli- signifies a thousandth whether it is attached to a gram, a liter or a meter.
Also known as
Used in a sentence
The dose was recorded as 5 milligrams given by mouth once daily.
Good to know
A milligram is a thousand times a microgram, and confusion between the two abbreviations is a recognized source of serious dosing errors, which is why microgram is often written out in full.
Word origin
Where “milligram” comes from
Milli- comes from the Latin mille, meaning thousand; gram reaches English through French from the Greek gramma, a small weight.