APOTHECARY SYSTEM

The apothecary system of measurement is an English system that is considered to be one of the oldest systems of measure. It is also referred to as the fraction system because parts of units are expressed by using fractions, with the exception of the fraction one half, which is expressed as ss or SS¯. The notations in this system are unusual and can be confusing. The recent trend is to eliminate apothecary measures from use and to use the metric system. The Institute for Safe Medication Practices (ISMP) has recommended that all medications be prescribed and calculated with metric measures. The unusual notations and inclusion of fractions and Roman numerals have caused concern about use of the apothecary system.

Apothecary measures are now rarely found on medication labels except for labels on older medications that have been in use for many years (e.g., aspirin, phenobarbital, nitroglycerin, atropine). If a medication label does contain an apothecary measure, it is usually in conjunction with the metric equivalent. For example, the label on a bottle of Nitrostat (nitroglycerin) tablets shows that the tablets are labeled in milligrams, which is a metric measure, and also in grains (in parentheses), which is an apothecary measure. Figure 7-1 illustrates the use of metric and apothecary measures on medication labels. However, many of the newer labels do not include apothecary measures.

Although the metric system is the preferred system, apothecary measures, on rare occasions, are still seen on syringes (minim) and medication cups (dram); however, use of these measures has been discouraged. Many of the newer syringes in use today do not have the minim scale on them because of its inaccuracy. Although the use of the apothecary system is minimal, in the rare circumstances when its use is necessary, those involved in the administration of medications should be familiar with it, because safety in medication administration depends on understanding of the information on medication labels and medication orders. The text focuses on the common apothecary measures used and their metric equivalents. However, apothecary measures should always be converted to metric measures.

Apothecary Units of Measure

Historically, minims and drams, which are units of the apothecary system, were used in medication administration. Their use is now discouraged in medication administration. You only need to be able to recognize them because some syringes may still have a minim scale identified on them and a medicine cup may still have drams indicated on it. Minim is abbreviated with a lowercase “m”; dram is abbreviated as dr, and the symbol is a single-headed Z with a tail (3). A dram is equal to 4 mL.

Some apothecary measures are also used as household measures. Example: pint (pt) and quart (qt). Because the nurse encounters them in everyday situations, you need to know that 1 pint = 16 fluid ounces and 1 quart = 32 fluid ounces or 2 pints.

The common apothecary units used for medication administration are grain (gr) and ounce (, oz).

WEIGHT

The only one apothecary measure used for weight with which you need to be familiar is grain. The basic unit for weight is the grain.

Grain is abbreviated in lowercase letters as gr.

CAUTION

The abbreviation for grain (gr), an apothecary measure, is often confused with gram (g), a metric measure. Remember these two measures are different; when in doubt seek clarification.

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