The "grain" is an older method or measuring weight, from the outdated "apothecary" system of weight measurements. The "grain" was originally based on some agreed upon weight of a single grain of a certain plant (and therefore not universally reliable). However, you may still see the abbreviation "gr." on some medication bottles, next to or underneath the more modern equivalent in milligrams (mg). The modern equivalent is as follows: 1 grain = 65 mg (milligrams) (although 60 mg or 64.8 mg is sometimes used to equal "1 grain" for certain prescription medications). For non-prescription drugs such as Tylenol (acetaminophen) or aspirin: 1 grain = 65 mg therefore 2 grains = 130 mg
5 grains = 325 mg (one "regular strength" adult Tylenol or aspirin tablet in the USA) 10 grains = 650 mg A few other medications that have been around a long time, such as sodium bicarbonate tablets or phenobarbital tablets, are still still sometimes labeled with both mg and grain equivalents. These are simply hold-overs from decades ago when physicians wrote prescriptions using apothecary weight measurements. In modern healthcare, "grains" or "gr" as a measurement unit should never be used, for safety reasons. Measurement should be given in grams, milligrams, micrograms, or other appropriate metric measurement. Medication mistakes have happened because a physician's abbreviation of "gr" may be interpreted as either "gram" or "grain"... which are vastly different weight amounts!
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