Medication Safety
Institute for Institute for Safe Medication Practices, Horsham, PA
Sildenafil is used to treat pulmonary arterial hypertension (PAH) in both adult and pediatric patients. The recommended oral dose is 10 mg (for pediatric patients weighing 20 kg or less) or 20 mg three times a day (for adult and pediatric patients weighing more than 20 kg). To facilitate use in the pediatric population, an oral suspension (10 mg/mL once reconstituted) packaged with a 2 mL oral syringe is available. However, because only 0.5 mL (5 mg) and 2 mL (20 mg) dose markings are shown on the syringe, dosing errors can occur for patients requiring a 10 mg dose.
In addition, infants are often dosed based on weight, (e.g., 0.25 mg/kg/dose every 6 hours or 0.5 mg/kg/dose every 8 hours), introducing the risk that caregivers may try to use the enclosed syringe even though it does not have the appropriate dose markings.
In a recent case, an infant was prescribed sildenafil oral suspension with a dose of 0.5 mg/kg/dose which equated to 2.5 mg or 0.25 mL every 8 hours. While the pharmacy label included instructions in terms of the volume to be administered, the patient’s after-care summary included both the weight-based (2.5 mg) and volumetric (0.25 mL) doses. Also, when the pharmacy dispensed the oral suspension, it did not remove the enclosed syringe, which could not measure 0.25 mL. At home, the patient’s family administered 2.5 mL rather than 2.5 mg, resulting in a 10-fold overdose (25 mg/2.5 mL).
This is not the first time ISMP has received a report in which a patient or caregiver has confused a weight-based dose for a volumetric dose when both are presented on the pharmacy label or in after-care summaries. However, it is also possible that the syringe contributed to the confusion as there are no markings for 0.25 mL, but caregivers could use the syringe twice to administer 2.5 mL (one 0.5 mL dose plus one 2 mL dose). The error was not discovered until the family requested a refill that was too early based on the volume originally dispensed. The patient was monitored without any adverse effects noted.
Safe practice recommendations
It is critical that pharmacies review the dosing devices that come with manufacturer products and those that they purchase separately. When dispensing an oral liquid, provide the most appropriate device to measure the dose, that is, a dosing device that most closely matches the prescribed dose volume and limits the number of fills needed to administer one dose. In this case, for doses less than 1 mL, pharmacies should provide 1 mL oral syringes.
Also, keep in mind that the patient instructions printed on the pharmacy label should include the dose in the unit of measure used for administration, which in this case would be mL. Printing both the weight and volume dose on the pharmacy label can increase the risk of confusion for the patient. Use the teach-back method to teach patients and/or caregivers how to measure and administer this medication to verify their understanding. ■