How medications are ordered differs among physicians, drugs, and health care agencies. Some agencies require physicians to order generic products or only those drugs stocked by the pharmacy. Most drugs are ordered using the metric system, but occasionally another measurement system will be used; for example, heparin, a powerful blood thinner, is available in units. There are many reasons why medication orders appear in a variety of forms, and the nurse must be prepared to understand them all.

Calculation Methods

Calculating dosages involves the following three steps:

1. Determine whether the drug dosage desired (what is written in the physician’s order) is in the same measurement system as the drug dosage available. If they are not in the same measurement system, convert between the two systems.

2. Simplify by reducing to the lowest terms whenever possible.

3. Calculate the dosage quantity to be administered. This may be done by using fractions, ratios, or proportions. All of these calculation methods arrive at the same answer. It’s simply a matter of finding which method works best for you as the nurse.

4. To avoid errors in your calculations:

• Do not use “trailing zeroes.” In other words, write 2 g not 2.0 g. This is because a big error could be made if the decimal point is not seen.

• Write a zero in front of the decimal point if the answer is less than one. In other words, write 0.25 mg versus .25 mg. Again, the problem is that the dose may be wrong if the decimal is not obvious.

Fraction Method

When using fractions to compute drug dosages, write an equation consisting of two fractions. First, set up a fraction showing the number of units to be given over x, the unknown number of tablets or milliliters. For example, if the physician’s order states, “ibuprofen 600 mg,” you would write . On the other side of the equation, write a fraction showing the drug dosage as listed on the medication bottle over the number of tablets or milliliters. The ibuprofen bottle label states, “200 mg per tablet,” so the second fractionis . The equation then reads: