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Drug Dosage Calculations


The following terms are used in connection with medication doses:

Therapeutic dose: Also referred to as the normal adult dose, the usual dose or average dose, it is the amount needed to produce the desired therapeutic effect.

Dosage range: A term that applies to the range between the minimum amount of drug and the maximum amount of drug required to produce the desired effect.

Minimum dose: The least amount of drug required to produce a therapeutic effect.

Maximum dose: The largest amount of drug that can be given without reaching the toxic effect.

Toxic dose: The least amount of drug that will produce symptoms of poisoning.

Minimum lethal dose: The least amount of drug that can produce death. 

Medication Dosage Calculation:


If you are trying to find out what a milligram actually might look like, take a raisin, cut it into 1000 equal parts. There--each little part will weigh about 1 milligram! There are 453,592.37 milligrams in a pound. The fact that most drugs are measured in milligrams should alert you to realize that sometimes the most miniscule amounts of a substance can be very powerful. Label instructions should be followed very carefully.

Often, when the doctor orders a medication and requests that the patient receives the initial, or whole dose of the prescribed drug before leaving the office it is the responsibilty of the medical assistant to draw up, or count out the right amount and administer it to the patient, followed by charting the process into the patient's chart. Also, there are patients that come to the medical office for the sole purpose of receiving a dose of a prescribed medication, such as allergy shots, Vitamin B 12 injections, flu shots, without seeing the doctor that day. This will require a standing prescription order in the chart form the doctor, and again, is usually carried out by the medical assistant.

Factors Affecting Drug Effect:

The two primary factors that determine or influence the dose are age and weight; but there are more:

  • Age
  • Weight
  • Gender
  • Time of administration
  • Immune response
  • Tolerance
  • Accumulation
  • Pathological factors
  • Psychological factors

Other factors:

  • Genetic make-up (nationality, ethinic heritage)
  • Occupation
  • Habitual use
  • Frequency of administration
  • Mode of administration


Problem is, sometimes the dosage or medication's strength is not the same as the strength that you have on hand. In other words the doctor orders 500 mg of a certain medication in tablet form, you go to get the medication storage area and find it on the shelf, but when you check the label it is not the exact same strength as ordered. The only tablets you have on hand are 250 mg strength.

What to do next? The answer is rather simple, you must calculate the correct dose! In order to calculate the correct dose, you need to use the correct formula! The following formula to calculate dosage is easy to remember and if used properly it delivers the correct result in every instance:

Desired dose

x quantity of on-hand dose = desired dose


(That's: Desired dose divided by on-hand strength, multiplied by the quantity of on-hand dose (e.g.. 1 tablet), equals desired dose)

On-hand strength

For example, if a physician orders 500 mg of ibuprofen for a patient (which is the desired dose,) and you have 250 mg tablets (1 tablet = 250 mg) on-hand, the medical caregiver needs to dispense two tablets, because 500 divided by 250, then multiplied by 1 (one tablet) equals the desired dose, which comes to 2 (two tablets).


x 1 (tablet)
= 2 tablets


(That's: 500 divided by 250, multiplied by 1 tablet, equals 2 tablets)


This formula works with any other type and strength of medication, whether tablets, suppository, or liquid (drops, suspensions, syrups etc.)

Had it been the other way around and the doctor had ordered 250 mg of ibuprofen but all you had on hand was 500 mg tablets, your calculation would look like this:


x 1 (tablet)
= 0.5 tablets


(That's: 250 divided by 500, multiplied by 1 tablet, equals half of a tablet)


It even works for a liquid medication, for example where 1 cc (liquid) delivers 500 mg of a drug. If the doctor ordered 1500 mg of the drug you calculate:


x 1 cc = 3 cc


(That's: 1500 divided by 500, multiplied by 1 cc, equals three cc liquid)


 Remember: 1 cc is the exact same amount as 1 ml!


More Calculations—Pediatric Rules

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