Compounding an Oral Suspension for a Child
A paediatric ward requests an oral suspension of a drug that is only available as 25 mg tablets. The child weighs 18 kg and the prescribed dose is 3 mg/kg twice daily for 10 days. The pharmacy has no commercial suspension and no ready-made vehicle suitable for the child. The pharmacist proposes crushing tablets, dispersing them in a suspending agent, and adding a sweetener. The available tablets contain the active drug plus lactose and a small amount of magnesium stearate. The pharmacy has a calibrated electronic balance (readability 1 mg), a mortar and pestle, a graduated cylinder, and amber bottles. The pharmacist has not compounded this particular preparation before and there is no validated in-house formula. The ward wants the first dose within two hours.
1. Calculate the dose per administration and the total quantity of drug needed for the course. What practical problems arise from using 25 mg tablets to measure a dose of this size?
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Dose = 3 mg/kg x 18 kg = 54 mg twice daily. Total for 10 days = 54 mg x 2 x 10 = 1080 mg. Each 25 mg tablet contains 25 mg, so 1080/25 = 43.2 tablets are needed in total, i.e. about 2.16 tablets per dose. The problem is that a single dose of 54 mg cannot be measured exactly from whole 25 mg tablets: 2 tablets give 50 mg (7.4% under) and 2.16 tablets is not a practical unit. Splitting tablets to get 0.16 of a tablet is inaccurate and unverifiable. A better approach is to prepare a bulk suspension of known concentration, for example 25 mg/5 mL by dispersing a known number of tablets in a known volume, then measure the dose volumetrically. Even then, the excipients (lactose, magnesium stearate) are being co-administered and magnesium stearate is insoluble and can cause foaming or caking. The calculation must be checked by a second person and the final concentration stated on the label.
2. The ward wants the dose in two hours and there is no validated formula. What should the pharmacist do, and what are the risks of proceeding as proposed?
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The pharmacist should not improvise an unvalidated extemporaneous preparation for a child when a licensed alternative might be obtainable, and should first check whether an imported or specials-manufactured suspension, or a different formulation of the same drug, can be sourced. If compounding is unavoidable, the pharmacist needs a documented formula with a stated concentration, a suitable vehicle, a beyond-use date, and a method that can be reproduced and checked. Risks of the proposed approach: unknown stability of the drug in the chosen vehicle; uneven distribution of drug in the suspension leading to under- or over-dosing (a serious problem with a narrow-therapeutic-index drug); inaccurate measurement from tablet splitting; microbial contamination if no preservative and no clean technique; and no validated assay to confirm content uniformity. The two-hour deadline is a service pressure, not a clinical justification for skipping verification. The pharmacist should communicate the delay, offer the first dose from a verified source if possible, and escalate to a senior pharmacist or the on-call medicines information service.
3. Assuming a validated formula is found, what labelling, record-keeping, and counselling points are essential?
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Label: drug name, concentration in mg per mL (not just mg per 5 mL), total volume, dose in mL with a suitable oral syringe, frequency, duration, storage conditions, beyond-use date and time, batch or preparation number, and 'shake well'. Record: the formula used and its source, the identity and quantity of ingredients, the tablet batch numbers, the vehicle and preservative, the final volume, the preparer and the checker, and the date and time of preparation. Counselling: demonstrate measuring with the oral syringe rather than a kitchen spoon; explain shaking before each dose; state what to do if a dose is missed; warn about visible changes (colour, smell, clumping) that mean the preparation should not be used; and confirm the child's weight used for the dose. Advise the ward to report any adverse effect. The pharmacist should also arrange follow-up to confirm the child is tolerating the preparation and that the dose is being given accurately.