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

6 thoughts on “Pharmaceutical Prescriptions, the Metric System and Your Safety (a Guest Post)

  1. The design of everyday things is of paramount importance to their ease of use. Unfortunately there is often very little thought that goes into them or unintentional bias that ends up in the design of everyday things. They are not thought of as important when compared with “the new, new thing.” Often their design is only an afterthought, and seen as an unimportant task. In the case of dosage measures, it can possibly mean the difference between life and death, but designers seldom win awards for over the counter measuring cups with high affordance and clarity of dosage. That’s not good. Very nice write-up.

  2. A comment on Easter Sunday about metrication.

    While we agree that the tablespoon (properly abbreviated Tbsp.) and teaspoon (properly abbreviated tsp.) are not good medicine dosages, we disagree that only metric foots the bill.

    Many of the situations, such as using a full dosing cup, could happen with metric measures as well.

    The proper units are dra(ch)ms, ounces apth. (same as the troy ounce), fluidrams, fluid ounces.

    The comment that the liquid measures for apothecary and US liquid are different is incorrect. They are the same sizes and capacities.

    The proper abbreviation for the grain is Gn. not gr. Since gr. is ambivalent, it should be used neither for grains nor for grams. Gn. and gr. are proper.

    Poor labeling and handwriting kill patients, not units of measurement. Micrograms and milligrams being mixed up is far more dangerous than any apothecary dosing error, as that situation would entail a thousand-fold error.

  3. There is no doubt that pharmaceutical prescriptions, and labels when written with metric measures, have reduced errors in medication. Most errors are caused by the user, or the parent, dispensing the medication. Most people are unfamiliar with drams and grains, as they are with milliliters and grams, and as has been noted teaspoons, and tablespoons, have a wide variations in quantity. Education is the key, as it is for most metric problems. If the law was changed, to make dispensing cups, with clear milliliter markings manditory for liquid medication, and clear instructions on how to uses the cup, then I’m sure that many errors would be avoided. This would also enable the user to become more familiar, with metric measures by knowing how much 5 mL, 10 mL, or 25 mL is when dispensing. 
    Metric education by using the cup will help reduce errors.
    Thanks for another interesting article from your site.

  4. Its important, when dispensing medication, to use a measurement system that has no variations.
    The milliliter (mL), and the gram (g), have no variations. They are the same value and quantity worldwide.
    The dram is confusing because it has a least four values.
    In the Avoirdupois system, the dram is equal to 1.771 845 195 312 5 grams.
    In the Apothecaries system, the dram is equal to 3.887 934 6 grams.
    In the USC system, the fluid dram is equal to 3.696 697 109 375 milliliters.
    In the British Imperial system, the fluid dram is equal to 3.551 632 812 5 milliliters.
    The metric system has a number of advantages over both USC and Imperial. One of the advantages is that its values of weights and measurements are consistent worldwide with no variation or double standard.

  5. Fluidrams and drams are two different units. And the dram is not used in avoirdupois anymore. Those units are always referred to as fractions of an ounce in modern labels.

    In the British Imperial system, a dram or fluidram are no longer a legal units.

    In any case, for the dispensation of medicine, the fluidram is the only unit that is really appropriate, anyway. For weights, grains are the only unit commonly used, and this would probably be the case even past the point of 20- or 60- grains.

    To avoid confusion, dram avdp. or dram apth. can be used to distinguish. In writing of prescriptions, f3 (actually the symbol looks like a 3 with a flattened top and a second line angling down in a 7:30 position to the bottom loop) is always used.

    Since medicines are never dispensed in avoirdupois units there is no confusion. Nor are avoirdupois units indicated on dosing cups. Originally, apothecary and troy (basically the same system, just divided differently) were used instead of avoirdupois because, within allowable error, prescriptions were written in Latin and could be used with good success even allowing for these variances.

    Since apothecary’s sole use is in the United States (it was made illegal in the UK in 1971) this confusion no longer exists. . .

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