Various systems of measures commonly used in prescribing, compounding and dispensing practices

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The field of pharmacy involves various systems of measures to ensure accuracy and consistency in prescribing, compounding, and dispensing medications. These systems provide a standardized framework for healthcare professionals to communicate and work with precise measurements. In this comprehensive exploration, we will delve into the commonly used systems of measures in pharmacy, including the metric system, apothecary system, and household system.

1. Metric System:

The metric system, also known as the International System of Units (SI), is a decimal-based system of measurement that is widely used in science, industry, and various fields, including pharmacy. It provides a standardized and consistent approach to measuring quantities, facilitating communication and collaboration on a global scale. The metric system is based on a set of base units, prefixes, and multiples, making it easy to convert between different units.

Base Units of the Metric System:

  1. Meter (m): The unit of length. One meter is equivalent to 100 centimeters or 1,000 millimeters.
  2. Gram (g): The unit of mass. One gram is equal to 1,000 milligrams.
  3. Second (s): The unit of time.
  4. Ampere (A): The unit of electric current.
  5. Kelvin (K): The unit of temperature.
  6. Mole (mol): The unit of amount of substance.
  7. Candela (cd): The unit of luminous intensity.
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Metric Prefixes:

The metric system employs prefixes to denote multiples or fractions of the base units, which simplifies expressing measurements across a wide range of magnitudes. Some common prefixes include:

  1. Kilo- (k): 1,000 times the base unit.
    • Example: 1 kilogram (kg) is equal to 1,000 grams.
  2. Hecto- (h): 100 times the base unit.
  3. Deca- (da): 10 times the base unit.
  4. Deci- (d): 1/10th of the base unit.
  5. Centi- (c): 1/100th of the base unit.
    • Example: 1 centimeter (cm) is equal to 0.01 meters.
  6. Milli- (m): 1/1,000th of the base unit.
    • Example: 1 milligram (mg) is equal to 0.001 grams.
  7. Micro- (µ): 1/1,000,000th of the base unit.

Metric Units in Pharmacy:

  1. Volume:
    • The standard unit is the liter (L), and smaller volumes are expressed in milliliters (mL) or cubic centimeters (cc).
    • Example: A prescription might specify 10 mL of a liquid medication.
  2. Mass:
    • The gram (g) is the base unit for mass. Smaller quantities are expressed in milligrams (mg), and larger quantities may be in kilograms (kg).
    • Example: A medication might be prescribed as 500 mg.
  3. Length:
    • While length is not commonly used directly in pharmacy, it is indirectly involved in measuring dimensions for compounding and packaging.

Advantages of the Metric System in Pharmacy:

  1. Decimal Nature: The decimal nature of the metric system simplifies conversions and calculations, reducing the risk of errors.
  2. Global Standardization: The metric system is an internationally recognized standard, promoting consistency and facilitating communication between healthcare professionals worldwide.
  3. Precision: Metric units are designed to be precise and easily scalable, allowing for accurate measurement of both small and large quantities.
  4. Ease of Use: The metric system’s logical structure and consistent prefixes make it user-friendly for healthcare practitioners, contributing to efficient pharmacy practices.

Challenges and Considerations:

  1. Transition Periods: Some regions may still use alternative systems like the apothecary system, leading to the need for conversions and dual-labeling.
  2. Patient Understanding: While healthcare professionals may predominantly use the metric system, patients might be more familiar with household units, necessitating effective communication and education.

2. Apothecary System:

The apothecary system is an outdated system of measurement used in the field of pharmacy and medicine, primarily in the United States and Great Britain. It is also known as the apothecaries’ system. This system is not commonly used in modern healthcare practices, as it has been largely replaced by the metric system in most parts of the world. However, historical references and older medical literature may still use the apothecary system.

Key units in the apothecary system include:

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  1. Grain (gr): The basic unit of weight in the apothecary system. One grain is equivalent to approximately 64.79891 milligrams.
  2. Minim (m or ♏): A unit of volume, often used for measuring liquids. It is equivalent to approximately 0.0616 milliliters.
  3. Dram (dr): Equal to 60 grains or 3 scruples. It is used for measuring both weight and volume.
  4. Ounce (oz): The apothecary ounce is different from the avoirdupois ounce used in the modern metric system. An apothecary ounce is equivalent to 480 grains or approximately 31.1035 grams.
  5. Pound (lb): The apothecary pound is different from the avoirdupois pound. An apothecary pound is equivalent to 12 apothecary ounces.

While the apothecary system has historical significance, its use has largely been replaced by the metric system in modern medicine and pharmacy due to its simplicity and global standardization. In contemporary healthcare, medical professionals use metric units for both weight and volume measurements to ensure consistency and ease of communication across different regions and healthcare systems.

a. Volume:

The main unit for volume in the apothecary system is the fluid ounce (fl oz), and smaller quantities are expressed in fluid drams (fl dr). Despite its diminished use, you may encounter prescriptions specifying a certain number of fluid ounces or fluid drams of a liquid medication.

b. Weight:

The apothecary system uses the grain (gr) as the base unit for weight. Smaller quantities may be expressed in minims (m), and larger quantities in drams (dr) or ounces (oz).

c. Conversions:

Converting between units in the apothecary system can be more complex compared to the metric system. For example, 1 fluid ounce is equivalent to 8 fluid drams.

3. Household System:

The household system, also known as the household units or the customary system, is a system of measurement commonly used in the United States for everyday activities and commerce. While the metric system is widely used around the world, the household system persists in the U.S., particularly in informal contexts like cooking, home improvement, and some trade industries. The household system includes units for length, weight, volume, and temperature. Some key units in the household system include:

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  1. Length:
    • Inch (in)
    • Foot (ft)
    • Yard (yd)
    • Mile (mi)
  2. Weight:
    • Ounce (oz)
    • Pound (lb)
    • Ton (short ton)
  3. Volume:
    • Teaspoon (tsp)
    • Tablespoon (tbsp or T)
    • Fluid Ounce (fl oz)
    • Cup (c)
    • Pint (pt)
    • Quart (qt)
    • Gallon (gal)
  4. Temperature:
    • Fahrenheit (°F)

While the household system is still prevalent in certain areas of daily life in the United States, it’s important to note that the metric system is the standard for scientific and most professional applications globally. The metric system is considered more consistent and easier to use for scientific and technical purposes due to its base-10 nature. However, the household system remains deeply ingrained in American culture and continues to be used in various everyday contexts. Efforts to transition to the metric system in the U.S. have been discussed, but a widespread shift has not occurred as of my knowledge cutoff date in January 2022.

4. Specialized Units:

Household units for volume include teaspoons (tsp), tablespoons (tbsp or tbl), cups, pints, quarts, and gallons. It’s important to note that these measurements are not standardized, and the actual volume they represent may vary.

Specialized units refer to units of measurement that are tailored for specific purposes or industries. These units are often designed to address the unique needs of a particular field or discipline. In addition to these primary systems, some specialized units are used in specific contexts within pharmacy:

a. International Units (IU):

Used for measuring the biological activity of substances like vitamins and hormones.

b. Molarity (mol/L):

Expresses the concentration of a solution in terms of moles of solute per liter of solution. Commonly used in compounding and preparing solutions.

c. Units for Compounding:

In compounding pharmacies, unique units may be used, such as drops, scoops, or specific measurements related to compounding equipment.

5. Conversion Factors:

Pharmacists often need to convert between different systems, especially when interpreting prescriptions or compounding medications. Establishing conversion factors between units is crucial for maintaining accuracy. For example, when converting between milliliters and teaspoons, the standard conversion is 5 mL equals 1 teaspoon.

6. Importance of Standardization:

The move towards standardization, especially with the widespread adoption of the metric system, is driven by the need for accuracy, consistency, and global communication within the healthcare industry. Standardized units minimize the risk of errors in prescription interpretation and compounding.

7. Challenges and Considerations:

Despite the emphasis on standardized systems, challenges still exist. Prescriptions from different regions may use different units, and patient understanding of measurements may vary. Pharmacists must be vigilant in clarifying any ambiguities in prescriptions and communicating effectively with patients.

8. Technology in Measurement:

Modern pharmacy practices increasingly rely on technology to enhance precision and reduce errors. Automated dispensing systems, electronic prescribing, and digital medication administration records contribute to the accuracy of measurements and dosage calculations.


In the complex landscape of pharmacy, the choice of measurement systems profoundly impacts patient safety and treatment outcomes. The metric system stands out as the dominant and globally accepted system, providing a standardized framework for precise measurement. While historical systems like the apothecary system persist, the trend is clearly towards universal adoption of the metric system in pharmaceutical practice. Pharmacists must navigate these systems adeptly, ensuring accurate prescriptions, compounding, and dispensing of medications while embracing technological advancements that further enhance precision and reduce the risk of errors in healthcare delivery.

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