Stability of Nitrofurantoin in Extemporaneously Compounded Suspensions: DISCUSSION

The lack of a commercially available suspension of nitrofurantoin in Canada poses a problem for children and adults who are unable to swallow tablets and capsules. Until the time of this study, nitrofurantoin suspensions used at the authors’ institution had been extemporaneously prepared in a methylcellulose vehicle. This product had an expiration date of only 8 days when kept refrigerated. To the authors’ knowledge, there are no published stability studies on nitrofurantoin suspensions prepared in a 1:1 mixture of Ora-Sweet and Ora-Plus.

In the weekly analysis of samples, no notable change in colour was observed in the suspensions after storage at 4°C or 25°C for up to 91 days. Very slight changes in odour and taste were observed between days 14 and 21, but these characteristics remained stable for the rest of the 91-day period. Viscosity was constant. Precipitates were easily resuspended, and there was no caking or clumping of material. Although these measures are qualitative, observations were documented by the same individual throughout the 91 days, thus eliminating interpersonnel bias. Variation in pH was not notable.

A limitation of this study design relates to the freezing of the samples at -85°C until the time of batch analysis. It was assumed that nitrofurantoin would not degrade at this low temperature and that no volume losses would occur because of freeze-drying during storage. In addition, it was assumed that errors due to serial analysis would have been greater than errors occurring with batch analysis.
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According to qualitative, pH, and HPLC analyses of weekly samples, nitrofurantoin suspensions of 10 mg/mL stored at either 4°C or 25°C remained stable and maintained at least 90% of their original concentrations for up to 91 days. These results led to changes at the authors’ hospital for extemporaneous compounding of nitrofurantoin suspensions. The expiration date has been extended from 8 days to 3 months.


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