Stability of oral liquid penicillins in unit dose containers at various temperatures

The effect of freezing on the stability of reconstituted, unit dose packaged oral amoxicillin trihydrate and ampicillin suspensions and penicillin V potassium solutions was studied. Powders for suspension or solution of the three penicillins were reconstituted according to manufacturers' directions to yield a concentration of 250 mg/5 ml. Samples of 5 ml then were stored in amber, screw-cap, glass vials at 25 C, 5 C, 0 C, -10 C or -20 C for 5, 10, 20, 30 or 60 days. The concentration of active constituents remaining after storage was determined spectrophotometrically. All three penicillins retained at least 90% of their original activity for at least 60 days when stored at -20 C. Ampicillin and pencillin V potassium retained at least 90% of their activity after 60 days of storage at -10 C but amoxicillin concentration decreased to 88% and 87% of initial concentration after 60 days storage at -10 C and 0 C, respectively. Degradation appeared to be by a zero-order process for amoxicillin and ampicillin and by a first-order process for penicillin V potassium. It appears that oral solutions of amoxicillin, ampicillin and penicillin V potassium can be effectively stored for at least 50 days in a freezer at -10 C with little loss of activity.