Lecture 2: How to Write Drug Dosages

In order to write and read drug orders or prescriptions, we need to know how they should be written. Because it is so important to minimize the chance that someone might misread a drug order, there are specific units of measure, abbreviations, and rules for writing orders that must be used. In this section, we will learn what these rules are and practice using them in example drug orders.

The Metric System

The most commonly used system of measurement in the medical profession is the metric system. It is the most modern system and is preferred because all units of measure come in powers of 10, making it easy to convert from one unit to another.

Metric system units of measure :

weight length volume
grams g meters m liters L
milligrams mg 1000 mg = 1 g
micrograms mcg (μg) 1000 mcg = 1 mg
centimeters cm
milliliters mL 1000 mL = 1 L
(cubic centimeters cc) (1 cc = 1 mL)

In the above table we can see the basic units of measurement for each category, along with the abbreviations for each unit.

Some other metric units:

Most drug dosages are measured by weight in grams, milligrams or micrograms; however certain special drugs have other metric units that measure properties other than weight:

  • International Units (units): International Units, often just referred to as Units, are used to measure the power or action of a drug. The volume or weight of a Unit will vary from drug to drug; for example, in penicillin, which is an antibiotic, a substance which kills bacteria, the ability of the antibiotic is measured by it's power to kill bacteria. If one kind of penicillin is mixed so that 1 mL contains 200,000 U and another kind of penicillin is mixed so that 1 mL contains 1,000,000 Units, the second drug has five times more bacteria-fighting power than the first drug, even though one mL of each has the same volume.

    Drugs such as insulin, penicillin, and heparin, all of which we will encounter in this course, are all measured in Units; vitamins, hormones, and blood products in general are also often measured in units. However, it is very important to note that the volume or weight of a Unit of insulin is usually very different from a Unit of penicillin or a Unit of heparin. So while 1 mL of insulin may have 100 Units of insulin in it, 1 mL of heparin may have 10,000 Units of heparin in it!

    To read more about International Units, click here.

  • Milliequivalents (mEq): Milliequivalents measure the drug's ability to react with electrons. Drugs such as potassium chloride (KCl), calcium gluconate, sodium bicarbonate, and sometimes even sodium chloride can be measured in milliequivalents.
    To read more about Milliequivalents, click here.

Rules for Writing Drug Dosage Orders in the Metric System:

When we write dosage orders for drugs in the metric system, we have to follow a certain set of rules that have been devised to minimize the most common mistakes that people often make when reading drug orders. These rules are:

  1. The unit should come after the number.
    For example, to write five grams, we would write 5 g, not g 5.

  2. All numbers should be written using the Hindu-Arabic decimal system. This is just a fancy way of saying that we should use regular numbers (as opposed to Roman numerals).
    For example, to write three micrograms we should write 3 mcg, not iii mcg.

  3. We should always use decimals instead of fractions.
    For example, to write one and one half milliliters, we should write 1.5 mL, not 1 1/2 mL

  4. When writing decimals that are smaller than 1, we should always put a leading zero before the decimal point.
    (This is important because it removes the possibility that someone will misread .3 as 3 or .42 as 42.)
    For example, to write three tenths cubic centimeters, we should write 0.3 cc, not .3 cc.

  5. When writing decimals, we should leave off any unnecesary zeros after the decimal point.
    (This reduces the chances that someone could misread 1.50 as 150 or 20.0 as 200.)
    For example, when writing ten and one half kilograms, we should write 10.5 kg, not 10.50 kg.

 Common Mistakes to Avoid:

The Apothecary System

The traditional system of measurement used in the medical profession in the West has been the apothecary system, which has its roots in Latin. It is no longer commonly used because it contains script symbols that can be hard to read and because its units are much older and therefore more irregular, requiring more work to convert from one unit to another.

Apothecary system units of measure :

weight length volume
grains gr feet ft ounces
pounds lb inches in drams dr or
    minims min

In the above table we can see the basic units of measurement for each category, along with the abbreviations for each unit.

Rules for Writing Drug Dosage Orders in the Apothecary System:

When we write dosage orders for drugs in the apothecary system, we have to follow a certain set of rules that were traditionally used with apothecary abbreviations so that we minimize the chances that they could be confused with metric measurements. These rules are:

  1. The unit should come before the number.
    For example, to write five grains, we would write gr v, not gr v.

  2. All numbers should be written using Roman numerals. Notice that when Roman numerals are used in drug orders, they are usually lowercase and when the letter i is written, it is written with a line over the i instead of a dot, like this: ι. (This is because it is too easy to mistake ii for 11 if the i's are written in uppercase without dots or with regular dots that can be easy to miss.)
    (The use of Roman numerals makes sense because the apothecary system was based in Latin, where Roman numerals were the standard way of writing numbers.)
    For example, to write three minims we should write min ιιι, not min 3 .

  3. We should always use fractions instead of decimals.
    One special abbreviation for

    1
    2
    is ss. When writing apothecary dosages, one can write either
    1
    2
    or ss. For example, to write one and one half drams, we should write ι
    1
    2
    or ι ss, not 1.5

 Common Mistakes to Avoid:

The Household System

The Household system is not used in hospitals among staff; its major purpose is when writing out instructions for patients, so it includes the most common "household" abbreviations written so that an average non-medical person can understand them.

Household system units of measure :

weight length volume
pounds lb meters m ounces oz
  inches in tablespoon T or tbl
    teaspoon t or tsp
drops gtt

In the above table we can see the basic units of measurement for each category, along with the abbreviations for each unit.

Rules for Writing Drug Dosage Orders in the Household System:

When we write dosage orders for drugs in the household system, we have to follow a certain set of rules that have been devised to make them easy for the average patient to read and understand. These rules are:

  1. The unit should come after the number.
    For example, to write five drops, we would write 5 gtt, not gtt 5.

  2. All numbers should be written using the Hindu-Arabic decimal system. This is just a fancy way of saying that we should use regular numbers (as opposed to Roman numerals).
    For example, to write three tablespoons we should write 3 T, not iii T .

  3. We should always use fractions instead of decimals.
    For example, to write one and one half ounces, we should write 1

    1
    2
    oz, not 1.5 oz

 Common Mistakes to Avoid:

Examples of Correctly Written Dosages

Example:

Write one and one half grams using the correct rules.

Because the units used here are grams, we know that we must use the rules for the metric system.
So we must put the number before the units.
The metric system also requires that we use arabic numbers.
The metric system also requires that we use decimals instead of fractions.
We must be careful not to use any unnecessary zeros after the decimal point.

So, the correct way to write this is:

1.5 g

Example:

Write one and one half grains using the correct rules.

Because the units used here are grains, we know that we must use the rules for the apothecary system.
So we must put the number after the units.
The apothecary system also requires that we use roman numerals.
The apothecary system also requires that we use fractions instead of decimals.

So, the correct way to write this is:

gr ι

1
2

or

gr ι

ss

(because ss is one way to abbreviate one-half in the apothecary system)

Example:

Write one half milligram using the correct rules.

Because the units used here are milligrams, we know that we must use the rules for the metric system.
So we must put the number before the units.
The metric system also requires that we use arabic numbers.
The metric system also requires that we use decimals instead of fractions.
We must be careful not to use any unnecessary zeros after the decimal point.We must also sure that we put a leading zero before the decimal point, since this decimal is smaller than one, because we don't want anyone who reads it to mistake it for a whole number.

So, the correct way to write this is:

0.5 mg

Example:

Write one half microgram using the correct rules.

Because the units used here are micrograms, we know that we must use the rules for the metric system.
So we must put the number before the units.
The metric system also requires that we use arabic numbers.
The metric system also requires that we use decimals instead of fractions.
We must be careful not to use any unnecessary zeros after the decimal point.We must also sure that we put a leading zero before the decimal point, since this decimal is smaller than one, because we don't want anyone who reads it to mistake it for a whole number.

So, the correct way to write this is:

0.5 mcg

Example:

Write two and two fifths ounces using the correct rules.

Because the units used here are ounces, we know that we must use the rules for the apothecary system.
So we must put the number after the units.
The apothecary system also requires that we use roman numerals.
The apothecary system also requires that we use fractions instead of decimals.

So, the correct way to write this is:

ιι

2
5

Example:

Write two and two fifths drams using the correct rules.

Because the units used here are drams, we know that we must use the rules for the apothecary system.
So we must put the number after the units.
The apothecary system also requires that we use roman numerals.
The apothecary system also requires that we use fractions instead of decimals.

So, the correct way to write this is:

ιι

2
5