7
Calculation of doses
To perform calculations involving doses, it is essential to understand the terms ‘dose’ and ‘dosage regimen’.
Dose
A dose is the quantity or amount of a drug or drug formulation taken by, or administered to, a patient to achieve a therapeutic outcome.
The term ‘dose’ can be further qualified as a single dose, a daily dose, a daily divided dose, a weekly dose, as described with examples below:
- Single dose: for example, for self administration in and for treatment of acute anaphylaxis, the British National Formulary (BNF)-recom mended dose for adrenaline is 300 micrograms, i.e. a single dose of 300 micrograms
- Daily dose: for example, the BNF-recommended dose of atorvastatin is 10 mg once daily
- Daily divided dose: the recommended initial dose for sodium valproate is 600 mg daily in one or two divided doses, i.e. the dose could be given as one 600 mg dose or the 600 mg dose is divided into two doses of 300 mg each
- Weekly dose: mefloquine, when used as an antimalarial, has an adult dosage regimen of 250 mg once weekly.
The dose of a drug may be repeated. For example, in the case of mebendazole for the treatment of threadworm, the BNF states: ‘if re-infection [with threadworm] occurs a second dose may be needed after 2 weeks’.
Doses can also be calculated depending on the age, weight or body surface area of the patient. This is particularly true when reviewing the prescribing for children, and will be covered in Chapter 8.
Dosage regimen
A dose may be repeated regularly either throughout the day or following some other time period. Such scheduling of doses is called the dosage regimen.
Examples of different dosage regimens are:
- in the treatment of whipworm, the BNF-recommended dosage of mebendazole is 100 mg twice daily for 3 days, i.e. the dose is 100 mg of mebendazole and the dosage regimen is twice daily for 3 days
- erythromycin has a dosage regimen of 250–500 mg to be taken orally every 6 hours
- clarithromycin has a dosage regimen of 250 mg every 12 hours for 7 days.
Thus doses and dosage regimens vary with the drug and the disease/illness/symptoms that the drug is intended to treat. The dose and dosage regimen of a drug can be found in official publications, such as the BNF and in manufacturers’ literature.
Formulations
In addition, a drug may be presented in more than one formulation and in more than one strength. For example, the drug amoxicillin is available in the following formulations:
| capsules: 250 mg and 500 mg |
suspension:  |
sachets:  |
| injection: powder for reconstitution 250 mg, 500 mg, 1 g |
A prescription stating the dose as 250 mg amoxicillin could mean that the prescriber wants capsules, suspension or injection. In such a situation the pharmacist would need to make a judgement or query the prescriber’s intentions.
Doses based on units of formulations
Solid formulations
Examples of units of solid dosage forms are tablets, capsules, sachets and suppositories. The doses of such pharmaceutical products may be expressed as one or more units. Such doses may form part of a dosage regimen, e.g. two tablets at night for 3 days.
Some solid formulations may contain one or more active ingredients, e.g. Rifater tablets contain rifampicin, isoniazid and pyrazinamide, in which case the dose is normally expressed as units of the dosage form, e.g. six tablets daily.
Common examples of a number of active drugs in one dosage unit are the compound analgesics and compound antibiotics:
| co-codaprin (aspirin and codeine phosphate) |
| co-codamol (codeine phosphate and paracetamol) |
| co-dydramol (paracetamol and dihydrocodeine tartrate) |
| co-amoxiclav (amoxicillin and clavulanic acid) |
| co-fluampicil (flucloxacillin and ampicillin). |
The calculations involving such unit doses are relatively simple and are normally concerned with calculating a total number of dose units for a specific treatment period.
Some prescribers specify the number of days of treatment by using abbreviations. For example:
- days may be expressed as a fraction of a week, so 3 days will be represented as
and 5 days as 
- weeks may be represented as a fraction of the number of weeks in the year, so 2 weeks will be written as
and 6 weeks as 
- months may be represented as a fraction of the number of months in a year, so 6 months will be written as
and 9 months as 
. In the UK, when a month is used like this it generally represents 28 days
If a prescriber uses such an abbreviation, the calculation of the total number of dose units follows the same method as previously.
Oral liquid formulations
The normal unit of measurement for oral liquid medicines is the 5-mL spoon or multiples thereof. Some medicinal products have a dose or dose range larger than 5 mL. For example, magnesium trisilicate mixture has a dose range of 
three times a day. Such a dose range will need to be converted into units of 5 mL, i.e. two to four 5-mL spoonfuls three times a day.
With doses of oral liquid medicines it is important not to confuse 5-mL doses with the total volume of medicine to be supplied.
In some situations it may be necessary to calculate the length of time taken to use up the total volume of medicine supplied. Such a situation is illustrated in Example 7.4.
Dose volumes less than 5 mL
If a patient requires less than 5 mL as a dose, then measuring an accurate dose using a 5-mL spoon would be difficult. Such doses of less than 5 mL may be given using a 5-mL oral syringe. The 5-mL oral syringe is calibrated into 0.5 mL divisions from 1 mL to 5 mL.
Thus, it is possible to deliver volumes between 1 and 5 mL using an oral syringe. The volumes delivered by an oral syringe are usually used for children or very frail adult patients. Example 7.5 demonstrates the calculation of the total volume to be dispensed when a dose volume of less than 5 mL is prescribed.
Doses based on the weight of drug
Solid formulations
The dose of a drug, in most official publications, is a specified weight or weight range, e.g. codeine phosphate: dose by mouth 30–60 mg. The stated dose may be accompanied by other instructions such as time intervals between doses, specified times of administration, length of treatment, etc. For example, the dose of codeine phosphate is further expanded to: by mouth 30–60 mg every 4 hours, when necessary, to a maximum of 240 mg daily.
Many drugs are available in one or more formulations and may be available in one or more strengths. For example, codeine phosphate is available as a 15-, 30- and 60-mg tablet, as an injection containing 60 mg per millilitre and as a syrup containing 25 mg in 5 mL. A pharmacist may therefore be required to convert a dose of a drug into an equivalent number of tablets, syrup or another dosage form.
On occasions, two strengths of the same medicinal product have to be used to obtain the correct dose. Example 7.7 demonstrates such a situation.
In Examples 7.6 and 7.7 only tablets have been used to illustrate the basic calculations. However, in practice similar calculations could involve suppositories, capsules, powders or other solid dosage forms.
Liquid formulations
Liquid formulations may be prescribed using the weight of the drug as the dose. For example, a prescription may state: 250 mg of paracetamol to be taken twice a day supplied as a suspension. Paracetamol oral suspension contains 250 mg of paracetamol in each 5 mL. Thus, each dose would be 5 mL and the patient would be instructed to take one 5-mL spoonful twice a day.
If the dose is expressed as the weight of drug, then, in the case of a liquid formulation, the dose taken by the patient may be 5 mL or a multiple thereof. Example 7.8 illustrates such a situation.
If a liquid formulation is prescribed for a very frail person or a very young child, the dose required may be less than 5 mL. Example 7.9 demonstrates such a situation.
