Errors in dispensing and administering medication are common, and it is estimated that 2 percent of hospital admissions result in an adverse drug event. Although the majority of these cases do not cause great harm, some errors are fatal. A calculation is required to determine the patient's dosage from general dosage information or when the prescribed dosage does not match the available dosage. Here's how to calculate drug dosages.

Find the dosage formula for the medication. Drug dosages are frequently given in mg/kg units. This means milligrams (mg) of medication per kilogram (kg) of body weight. The patient's weight in kilograms also is needed for this example.

Determine the patient's dosage by multiplying the general dosage by the patient's weight. For example, if the general dosage is 25 mg/kg and the patient weighs 20 kg, the patient should be given 25 mg/kg x 20 kg or 500 mg.

Calculate the weight required for an adult dosage. Some medications have a maximum dosage regardless of the patient's weight. The minimum weight required for an adult dosage is given by dividing the maximum dosage by the general dosage.

Suppose we have a maximum dosage of 650 mg for the example in Step 2. Divide the maximum dosage, 650 mg, by the general dosage, 25 mg/kg, so a patient weighing 26 kg would receive the adult dosage.

Consider a situation where the prescribed dosage is different from the available dosage. The correct number of pills may be given by dividing the required dosage by the available dosage. For example, if the doctor has prescribed 500 mg of a medication to be taken and you have 250 mg pills, then you would need to take 500 mg/250 mg, or two pills.