1.11: Dosage Calculations

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A medicine can be more harmful than helpful if it is not taken in the proper dosage. A dosage (or dose) is the specific amount of a medicine that is known to be therapeutic for an ailment in a patient of a certain size. Dosages of the active ingredient in medications are usually described by units of mass, typically grams or milligrams, and generally are equated with a number of capsules or teaspoonfuls to be swallowed or injected. The amount of the active ingredient in a medicine is carefully controlled so that the proper number of pills or spoonfuls contains the proper dose.

Most drugs must be taken in just the right amount. If too little is taken, the desired effects will not occur (or will not occur fast enough for comfort); if too much is taken, there may be potential side effects that are worse than the original ailment. Some drugs are available in multiple dosages. For example, tablets of the medication levothyroxine sodium, a synthetic thyroid hormone for those suffering from decreased thyroid gland function, are available in 11 different doses, ranging from 25 micrograms (µg) to 300 µg. It is a doctor’s responsibility to prescribe the correct dosage for a patient, and it is a pharmacist’s responsibility to provide the patient with the correct medicine at the dosage prescribed. Thus, proper quantities—which are expressed using numbers and their associated units—are crucial for keeping us healthy.

Effects are dose-dependent

Chemicals are the most common things for which doses are measured, but there are others, such as radiation exposure. For humans, most doses of micronutrients and medications are measured in milligrams (mg), but some are measured in micrograms because of their potency. Nonmedicinal poisons span the measurement scale; some poisons are so dangerous that a single microgram of it could be deadly, whereas other substances take much more. For example, even water is toxic when consumed in large enough quantities.

Dosage (the size of each dose) determines the strength and duration of the health benefits of nutrients, and also of the therapeutic effects of medical treatments. Dosage also determines the severity of adverse effects of treatments and toxins.
Duration of exposure, that is, the period of time over which the dose was received (all at once or gradually) also determines its effects (the body may build tolerance to gradual exposure to a drug, while a large immediate dose could be deadly).
The route by which a dose is exposed to, may affect the outcome, because some medications have different effects depending on whether they are inhaled, ingested, taken transdermally, injected, or inserted.

The dosage, route, concentration, and division over time may all be critical considerations in the administering of drugs, or in responding to exposure to a toxin. In nutrition, the route is usually a given, as nutrients are generally eaten; while dosage and the frequency of ingestion of nutrients are very important variables in preventing disease and promoting overall health.

Calculating drug dosages for humans based on the doses used in animal studies can be based on weight (e.g., mg/kg) or surface area (e.g., mg/m²) based on weight. Desirable dosages may also vary among different persons according to body weight and other factors.

Biological agents

Biological agents (bacteria, viruses, parasites) may have different dosage units. This is because it is the ability of the organism to cause effects that is the important unit, not a specific quantity by weight, volume, or even numerical count. Often, the unit used is CFU (colony forming units), which is proportionate to the number of organisms present multiplied times the number able to reproduce on a culture medium such as a Petri dish.

Toxicology

In the realm of toxicology, several measures are commonly used to describe toxic dosages according to the degree of effect on an organism or a population, and some are specifically defined by various laws or organizational usage. These include:

LD50 = Median lethal dose, a dose that will kill 50% of an exposed population
NOEL = No Observed Effect Level, the highest dose known to show no effect
NOAEL = No Observed Adverse Effect Level, the highest dose known to show no adverse effects
PEL = Personal Exposure Limit, the highest concentration permitted under US OSHA regulations
STEL = Short-Term Exposure Limit, the highest concentration permitted for short periods of time, in general 15–30 minutes
TWA = Time-Weighted Average, the average amount of an agent's concentration over a specified period of time, usually 8 hours.

References

http://www.unc.edu/~bangel/quiz/quiz5.htm
http://cancerres.aacrjournals.org/cg...t/18/7/853.pdf "The Use of Body Surface Area as a Criterion of Drug Dosage in Cancer Chemotherapy" D Pinkel. Cancer Research 1958

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