| Reference | Drams (dr) | Micrograms (μg) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 2.82192 dr | 5000000 μg |
| One litre of water | 564.383 dr | 1 × 109 μg |
| Average adult human | 39506.8 dr | 7 × 1010 μg |
| A small car | 790137 dr | 1.4 × 1012 μg |
The dram is a unit of mass equal to one sixteenth of an avoirdupois ounce, or about 1.7718 grams. Its symbol is dr. There are 256 drams in a pound and 27.34375 grains in a dram, a set of numbers that shows how far the avoirdupois system drifted from tidy arithmetic.
The name descends from the Greek drachma through Latin and Old French, and the same root gives the modern currency of Armenia and the historical coin of Athens. The apothecaries' dram was a different and larger unit, one eighth of a troy ounce or 3.888 grams, and pharmacists used it until the twentieth century. Recipes and old medical texts therefore need reading with care, since an unqualified dram may be either.
Modern survival is narrow but real. Shotgun shells are the best-known case: the dram equivalent printed on a box of cartridges is a legacy figure describing the propellant charge in terms of the black powder that would have produced the same velocity, not the actual weight of modern smokeless powder. American recipes occasionally call for drams, and the unit persists in some descriptions of small hardware and fishing weights.
For everyday purposes the dram is now a curiosity, kept alive by its appearance on packaging and in older texts rather than by any working need. Where precision matters the same quantity is stated in grams, and where tradition matters the word survives without its arithmetic being examined too closely.
Two other drams complicate the picture. The apothecaries' dram, used in pharmacy until the twentieth century, was one eighth of a troy ounce and weighed about 3.888 grams, more than twice the avoirdupois dram described here. The fluid dram is a measure of volume rather than mass, one eighth of a fluid ounce, and it still appears on the graduations of old medicine glasses and on some American pharmacy equipment. A recipe or a formula written before the metric era therefore has to be read with its system identified first, because the same word can mean two different masses and a volume, and nothing in the notation distinguishes them.
One dram equals about 1.7718 grams, one sixteenth of an ounce, or 27.34375 grains.
The microgram is a unit of mass equal to one millionth of a gram, or one thousandth of a milligram. Its official symbol is µg, written with the Greek letter mu.
In medicine that symbol is a known hazard. Handwritten, µg is easily read as mg, a thousandfold error, and the mistake has killed patients. Safety bodies including the Institute for Safe Medication Practices therefore recommend writing mcg instead on prescriptions and labels, and most hospital systems now enforce it. The abbreviation is not SI and would be wrong in a scientific paper, but it is the safer choice on a drug chart.
Micronutrients live at this scale. Vitamin B12, folate, iodine, selenium, biotin and vitamin K are all dosed in micrograms, and daily requirements run from a few micrograms to a few hundred. Vitamin D is given either in micrograms or in international units, with one microgram equal to forty IU, a dual convention that causes constant confusion on supplement labels. Potent drugs use the unit too: fentanyl, levothyroxine, digoxin and most hormonal contraceptives are dosed in micrograms rather than milligrams.
Air quality reporting has made the microgram familiar to the public. Fine particulate matter is measured in micrograms per cubic metre, and the World Health Organization guideline for annual PM2.5 exposure is five micrograms per cubic metre. Nitrogen dioxide, sulphur dioxide and ozone limits are written the same way, so the number quoted in a news report about pollution is almost always a count of micrograms in a cubic metre of air.
Toxicology works here because potency and hazard both scale with dose. Digoxin is prescribed in a few hundred micrograms a day and becomes dangerous not far above that, thyroid hormone is dispensed in 25-microgram steps, and the opioid fentanyl is active at doses of tens of micrograms, which is why illicitly mixed powders containing it are so often lethal. Analytical chemistry meets the unit from the other direction: modern instruments detect pesticides, metals and pharmaceuticals in water at concentrations of micrograms per litre, and regulatory limits are written at that level. A microgram is invisible, weighs less than a speck of dust, and yet is the natural unit for both the dose that heals and the dose that kills.
One microgram equals 0.001 milligrams, one millionth of a gram, and one thousand nanograms.