Medicine and the Metric System

Allow me to present my main point upfront: we are endangering our health by not adopting the metric system in this country.

Prescriptions are written in metric units. Conversions (and possible errors) are made at the pharmacy.

Doctors write prescriptions in metric units. Conversions (and potential errors) are made at the pharmacy.

Let me offer up a couple of examples that hopefully makes this clear. It’s important to understand that the medical field depends on metric units (as does most of science, for that matter). If healthcare workers talk in metric units and the public at large rely on U.S. customary units there is bound to be confusion and misunderstandings. That’s best avoided where your heath is concerned since the consequences could be dire.

Metric unit dosing is more precise

Last year Pediatrics (Official Journal of the American Academy of Pediatrics) published an article called “Unit of Measurement Used and Parent Medication Dosing Errors.”1 One of the article’s bottom lines:

Parents who used milliliter-only unit made fewer dosing errors than those who used teaspoon or tablespoon units. Moving to a milliliter-only standard could reduce confusion and decrease medication errors, especially for parents with low health literacy and non-English speakers.

While a minor mistake (whether too much or too little medication) might not make a huge difference for an otherwise healthy adult, these errors can be magnified for babies or those whose health is already compromised.

Most of our teaspoons and tablespoons are meant for eating, not dosing

No on should use "silverware" as substitutes for measuring teaspoons and tablespoons for medicine to avoid dosing errors.

No on should use “silverware” as substitutes for measuring teaspoons and tablespoons for medicine if they want to avoid dosing errors.

Another issue brought up in the piece was that the use of teaspoon and tablespoon employed for liquid medicines “may endorse kitchen spoon use.” I don’t know about you, but I have three sets of measuring spoons and many more spoons that I use for eating commonly referred to as “teaspoons” and “tablespoons.” The problem is, it’s the eating spoons that are often used to measure medicines. (Yes, I used to do that too, without even thinking about it.) If you have a dosing cup with only milliliters, the potential for confusion is greatly reduced.

As if that’s not bad enough

Even your actual measuring spoons aren’t as precise as you think they are. At one point I came across information indicating that up to a 20 percent variance is allowed. Again, that 20 percent could cause dosing errors. In researching this article I came across a page called “Cooking for Engineers” with the post:

I’ve got three sets of measuring spoons, and their measurements differ from each other, up to 1/4 teaspoon! Is there a way to know which (if any), are accurate?

The suggestions that followed involved scales and the temperature at which one should measure the water used to determine volume. Too bad no one suggested going metric.

Additional endorsement of the metric system for health reasons

I also have a document from the Institute for Safe Medication Practices (ISMP) called 2014-15 Targeted Medication Safety Best Practices for Hospitals2 . Two of its best practices mention sole use of metric system units.

Best Practice 3: Measure and express patient weights in metric units only.

The rationale:

Significant medication errors have occurred when the patients’ weight is documented in non-metric units of measure (e.g., pounds) and it has been confused with kilograms (or grams). Numerous mistakes have been reported when practitioners convert weights from one measurement system to another, or weigh a patient in pounds but accidently document the value as kilograms in the medical record, resulting in more than a two-fold error.

Best Practice 5: Purchase oral liquid dosing devices (oral syringes/cups/droppers) that only display the metric scale.

ISMP has received more than 50 reports of mix-ups between milliliter (mL) and household measures such as drops and teaspoonfuls, some leading to injuries requiring hospitalization.

Beware teaspoon and tablespoon instructions on prescriptions

Almost uniformly, prescriptions are written in metric units. However, if you pick up a liquid prescription and the dose on the bottle is not metric (and in reads teaspoon and tablespoons), the pharmacy has had to make a conversion. Where there are conversions, there is the potential for mistakes.

In addition, one of the top six recommendations in the 15th annual report of the National Coordinating Council for Medication Error Reporting and Prevention3 includes a “Statement of support for use of the metric system to dose medications.”

Advocate for the metric system and help make the country a healthy place!



Notes: The article itself requires a subscription However, a summary is located here:

Pharmaceutical Prescriptions, the Metric System and Your Safety (a Guest Post)

This week’s post was written by a contact I have in Australia. He asked me about how much of a medicine dosing problem we have here (citing a previous post of mine) and I sent him some basic information. After doing some research on his own, he put together this very nice article that he’s given me permission to share with you. Please enjoy.

In the September 1902 issue of The Journal of The American Medical Association, “Meyer Bros, Druggist” writes in the Miscellany column:

…It is not necessary to collect souvenir spoons in order to find out the great discrepancy in the size of a spoon when compared to the regulation “one fluid dram” measure. Unfortunately, some of the dose glasses are as far from the mark as any teaspoon. If physicians and pharmacists will give the subject particular attention, they can educate the public up to the point of using only accurately graduated medicine glasses when taking liquid remedies.

In the next paragraph he is quoted advocating the metric system:

The Metric System a Necessity — We have pointed out, says Meyer Bros.’ Druggist, on various occasions that the metric system would be adopted by the United States Government just as soon as our commercial relations with the world at large reached that proportion which would make the adoption of the metric system a matter of necessity. It seems that our country has reached that point. The committee on coinage, weights and measures has decided to report favorably the Shafroth bill providing for the adoption of the metric system by the government of the United States.

“One fluid dram”
In the intervening 111 years how much has changed? Well, the good news is that the pharmaceutical industry switched to metric measurements in 1971, only 69 years after the Meyer Brothers recommended it.

Up until then pharmacists and doctors used U.S. customary weights and measures for recording patient’s heights and weights. For dispensing medicines they had two systems: the metric system (grams and milliliters) and a medieval system called Apothecary’s Measure in which weights were measured in grains, scruples and drams, and liquids were measured in minims, fluid drams, (the “regulation ‘one fluid dram’ measure” mentioned in the Miscellany column) fluid ounces, pints quarts and gallons. It all sounds as if it would be used at Hogwarts School of Witchcraft and Wizardry.

What is a grain? Apparently, a grain of wheat was a standard measure of weight. (It was later standardized at 64.799 milligrams.) Twenty grains make a scruple and three scruples are a dram (also written as drachm). Finally, eight drams equal one ounce, of which twelve ounces are a pound. Simple, isn’t it? For liquids, a minim is the volume of water weighing a grain, and of course, Apothecary’s ounces, pounds, pints, quarts and gallons are not the same as the U.S. customary measures with the same names.Grain

And to make sure we know it’s all medieval, apothecary’s measures were written using Roman numerals, so gr xx meant 20 grains.

Ron Aylor, a pharmacy technician in Georgia, has put a web site together to help train pharmacy technicians, and which includes a great explanation of Apothecary’s Measure and the symbols used to write them at:

This assumes that every physician actually used those arcane symbols and Roman numerals to write prescriptions and didn’t just write “grains” instead. How many mis-doses have been dispensed by pharmacists or clerks who couldn’t decipher a physician’s handwriting to tell if he wrote “1 gram” or 1 grain” on a prescription, or didn’t know if the abbreviation “1gr” was for “one grain” or “one gram”?

The only thing more difficult to understand than the Apothecary system itself is why its use persisted for so long. The British Pharmacopia converted to metric measurements in 1963 and the US Pharmacopia in1971. You can still buy medication with the active ingredients listed as grains. The generic aspirin I found for sale over the internet is one example and some non-prescription herbal remedies also have their ingredients listed in grains, probably to avoid any taint of modernity or science.

“The Metric System a Necessity”
Around 70,000 children are admitted to hospital each year in the USA because they have taken an overdose of medicine. The majority of them have sneaked into the medicine cabinet and helped themselves, but it is astonishing that 18% (about 12,600 per year) are given an overdose of medication by their parents. (Note 1)

This happens because the dosing instructions are unclear, the measurements are misread, the parents use inaccurate household teaspoons for measuring doses of medicine and the whole procedure is unfamiliar or done in a hurry because the baby is crying and the parent is distracted.

This problem has been known about and discussed for a long time. In the article quoted previously from the Journal of the AMA, Meyer Brothers make the suggestion that customers should be taught to use properly graduated measures for dispensing medicines. This simple and straightforward action has still not been implemented completely in the century since it was proposed.

Question: If you prescribed a precise dose of medication would you tell the patient to measure it with:
A) a common household implement, or
B) a special-purpose accurate measuring device?

Good news if you manufacture medicines: The Food and Drug Administration (FDA) says you can do either. Over-the-counter (OTC) and prescription medicines are prescribed using measures of milliliters (mL), tablespoons (tbsp) or teaspoons (tsp). A standard teaspoon holds 5 mL, a tablespoon holds 15 mL.

In practice, a teaspoon is designed as a table accessory, not as a precise measuring device for medication and can vary between 3 and 7 mL depending on the design. This is a source of error up to 40% from the prescribed dose and was noted in the column by Meyer Brothers.

In addition to the inaccuracy of the spoon there are several human errors:

•  The abbreviations tsp and tbsp maybe confused, leading to under-dosing if the customer uses teaspoons where they should use tablespoons, or overdosing if they do the reverse.

• Patients may not understand the dose size. They are prescribed a dose of 5mL but take five teaspoons when they get home.

• Patients are given a dosing cup which holds 20 or 30 mL and fill it completely thinking it is the volume of the dose.

An underdosing error reduces the effectiveness of the medication and may be as bad as not taking any medication at all, as the dose is just too small to work. An overdose from the wrong sized spoon or overfilling the measuring device can lead to major medical complications or even death.

Surveys have shown that as many as 70% of adults preparing a dose of medicine for children made one of the errors listed above. (Note 2) Patients with visual impairment, poor English-language skills or impaired abilities are obviously likely to do make these sort of errors more often than the rest of the population. It is especially important to get doses right for children as OTC medications for them are most likely to be in a liquid form to make it easy for the child to ingest.

It is vitally important that things should change, but the Food and Drug Administration has only voluntary guidelines for the design and markings of dosing devices and it is still permissible to prescribe doses of OTC medicines in teaspoons or tablespoons and not mention milliliters at all.

The Institute for Safe Medication Practices ( is promoting two measures to improve prescribing practices:

1) They recommend that all doses should be prescribed in metric measurements only and not in teaspoons or tablespoons.

2) They are asking pharmacists to instruct their customers in the use and cleaning of dosing devices such as cups and oral syringes, and to supply them to their customers if they are not included with the medication.


In Britain, to enable the public to get used to metric measurements, drug manufacturers supplied 5 mL plastic spoons to be given away with each bottle of medicine. This was a great help during the changeover to metric measurement there, and would be an inexpensive and effective way to get customers used to doses in milliliters here in the US.

Some manufacturers, but not all of them, supply dosing devices with their medications. These may be simple measuring cups and spoons or more sophisticated droppers or oral syringes. It is not mandatory to supply them and FDA guidelines for their marking and instructions are only voluntary. (Note 3)

Even when a measuring device is supplied with the medicine, if it is badly designed it can be confusing to the customer. The FDA guidelines document shows examples such as the label on the package listing a dose in teaspoons only, and the accompanying dosing cup marked with:Spoons

  • a scale of teaspoon, dessert spoon and tablespoon measurements all together
  • a scale of fluid ounces and,
  • a scale of milliliters

all of which would confuse anyone. Other errors they noted included stating a half-teaspoon dose or a 2 tsp dose on the packaging but not having a corresponding graduation on the cup.

All of these errors could be eliminated by a bit of careful thought at the design stage and some usability trials with customers.

Is it too difficult for an industry that makes profits of billions of dollars each year to make a simple device for measuring medicine doses, and to make the instructions and markings easy to understand? If the pharmacy companies are slow to act—on a problem that has persisted for over a century—why shouldn’t the FDA mandate a fix the problem?

Would it be too difficult to insist that all doses are in milliliters only? It would require minor changes to labels on bottles and packages, something that costs a few cents per bottle.

A national standard for describing and measuring doses would ensure a uniform method of measuring and dispensing medicines of all types, and would lead to customers performing the same procedure to dispense the medication, no matter what brand of medicine they purchased.

What would it cost? How many anxious visits to the hospital emergency room could be avoided?

Peter Goodyear


Note 1 –
Note – 2 Parents’ Medication Administration Errors

Role of Dosing Instruments and Health Literacy


Note 3 – Guidance for Industry – Dosage Delivery Devices for Orally Ingested OTC Liquid Drug Products – FDA May 2011