Measuring Medication Levels, Potential Toxic Drugs

Potentially toxic drugs, drugs with narrow therapeutic indices and those with variable pharmacokinetics should be monitored regularly to optimise dosing, obtain maximum therapeutic effect, limit toxicity and assess compliance. Routine measurement is rarely warranted, but rather should be tailored to answering a specific clinical question, and is of most value in medicines with a narrow therapeutic index or where there is considerable individual variation in pharmacokinetics. Essential medicines for which there is evidence to support such monitoring include:


Measure serum levels at about 12 hours after the last dose – e.g. in the morning before that day’s first dose. Levels should be less than 1 mmol/L and should be checked regularly while on therapy, with more frequent monitoring in the elderly and frail.


When dosed based on body weight, peak levels will usually be adequate, e.g. 6 mg/kg/day in a single daily dose. Trough levels taken immediately before the next dose may be valuable in identifying potential toxicity before it manifests as deafness or renal impairment. Aminoglycosides are contra-indicated in renal impairment.

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Levels may be helpful to confirm poor adherence or to confirm a clinical suspicion of toxicity. Routine measurement in patients with well controlled seizures and no clinical evidence of toxicity, is not appropriate. Individual levels may be difficult to interpret – if in doubt, seek assistance from a clinical pharmacokineticist.


Medicines should be prescribed only when they are necessary for treatments following clear diagnosis. Not all patients or conditions need prescriptions for medicines. In certain conditions simple advice and general measures may be more suitable.

In all cases carefully consider the expected benefit of a prescribed medication against potential risks. This is important during pregnancy where the risk to both mother and foetus must be considered. All prescriptions should:

• be written legibly in ink by the prescriber with the full name and address of the patient, and signed with the date on the prescription form.

• specify the age and weight of the patient in the case of children

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• have contact details of the prescriber e.g. name and telephone number In all prescription writing the following should be noted:

• The name of the medicine or preparation should be written in full using the generic name.

• No abbreviations should be used due to the risk of misinterpretation. Avoid the Greek mu (): write mcg as an abbreviation for micrograms.

• Avoid unnecessary use of decimal points and only use where decimal points are unavoidable. A zero should be written in front of the decimal point where there is no other figure, e.g. 2 mg not 2.0 mg or 0.5 ml and not .5 ml.

• Frequency. Avoid Greek and Roman frequency abbreviations that cause considerable confusion – qid, qod, tds, tid, etc. Instead either state the frequency in terms of hours (e.g. 8 hourly) or times per day in numerals (e.g. 3x/d)

• State the treatment regimen in full: medicine name and strength dose or dosage dose frequency duration of treatment e.g. amoxicillin 250 mg 8 hourly for 5 days

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• In the case of “as required” a minimum dose interval should be specified, e.g. every 4 hours as required.

• Most monthly outpatient scripts for chronic medication are for 28 days; check that the patient will be able to access a repeat before the 28 days are up.

• After writing a script, check that you have stated dose, dose units, route, frequency, and duration for each item. Consider whether the number of items is too great to be practical for the patient, and check that there are no redundant items or potentially important drug interactions. Check the script is dated and that the patient’s name and folder number are on the prescription card. Only then sign the script, and as well as signing provide some other way for the pharmacy staff to identify you if there are problems (print your name, use a stamp, or use a prescriber number from your institution’s pharmacy.)

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