Lecture 2: How to Write Drug Dosages

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
Do NOT use μg for micrograms--use mcg instead!

centimeters cm

milliliters mL	1000 mL = 1 L
cubic centimeters (cc) are milliliters	(1 cc = 1 mL)
Do NOT use cc for cubic centimeters--use mL instead!

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

In this table we have included some older abbreviations for micrograms and milliliters (or cubic centimeters) that used to be common, but that should no longer be used. These outdated abbreviations are included because you may still encounter them on drug labels or syringes, and because it is important to be aware when reading drug orders that some doctors may use outdated abbreviations--if you see any of these abbreviations when reading a drug order, it is a good idea to question the drug order, just to be sure!

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:

The unit should come after the number.
For example, to write five grams, we would write 5 g, not ~~g 5~~.
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~~.
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~~
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 centimeters, we should write 0.3 cm, not ~~.3 cm~~.
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 L for liters or in mL should always be capitalized: L or mL (not l or ml)
Be careful not to confuse the abbreviations mg for milligrams with the abbreviation mcg for micrograms.
When writing the abbreviation for micrograms, always write mcg instead of μg. You should be able to recognise that the abbreviation μg stands for micrograms because it is often used on printed labels, but when handwritten, μg can too easily be mistaken for mg and should never be used!
When writing the abbreviation for milliliters, always write mL instead of cc for cubic centimeter, even though milliliters and cubic centimeters are the same. You should be able to recognise that the abbreviation cc stands for cubic centimeter which is the same as a milliliter because you may still see it on printed labels or hear the term used in hospitals, but when handwritten, cc can too easily be mistaken for zeros and should never be used!
When writing the abbreviation for Units, always write units instead of U. You should be able to recognise that the abbreviation U stands for units because you may still see it on printed labels, but when handwritten, cc can too easily be mistaken for zeros and should never be used!
Even when rounding, you should still leave off any unnecessary zeros after the decimal point. So, for example, if you were told to round 10.01 to the nearest tenth, you would write 10, and never 10.0. In a chemistry or biology class you may have learned a different practice - in laboratory sciences the opposite rule applies, because in laboratory sciences we are concerned with indicating exactly how precise a number is - but in a clinical setting, our primary concern is to minimize the possibility of a number being misread, and this means leaving off any zeros at the end of the decimal part, even when rounding.

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.

In the US this system has been largely discontinued; however, some drugs labels still have some of these units printed on their labels, and occasionally you make work with a doctor who still writes some drug orders in this system, especially for particular drugs that have been traditionally been measured using this system (for example, certain dessicated thyroid medications and asperin have both traditionally been measured in grains). Because of this, it is important that you are aware of this system so that you can identify if a drug order has the potential to be misinterpreted. If you see a drug order written using the apothecary system, it would be reasonable to question it to be sure that you are interpreting it correctly.

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:

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

Be careful not to confuse the abbreviations for ounces with the abbreviation for drams.
Be careful not to confuse the abbreviations g for grams (or mg for milligrams) with the abbreviation gr for grains.

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	inches in	ounces oz
		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:

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

The abbreviation gtt for drops seems strange because it does not share any of the same letters (this is because it comes from Latin), so be careful, and don't confuse the abbreviation gtt for drops with dr for drams!
Be careful not to confuse the apothecary abbreviation of for ounces with the household abbreviation oz for ounces - when writing instructions for a patient, one should use oz, not , because it is not in common usage among the general public.

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