Why they did it
This study sought to determine whether doxycycline suspension made from doxycycline tablets maintains the appropriate concentration after storage for four weeks.
What they did
The researchers made a suspension by mixing crushed 100-mg film-coated doxycycline hyclate tablets with a 50:50 mixture of syrup and suspension vehicles, which met the compendial requirements of the U.S. Pharmacopeia (USP) National Formulary. Two formulations were prepared (33.3 mg/ml and 166.7 ml/ml), and three batches of each strength were made and stored at either room temperature (71.6 to 78.8 F [22 to 26 C]) or a controlled cold temperature (refrigerated 35.6 to 46.4 F [2 to 8 C]). All preparations were protected from light.
Researchers measured doxycycline concentrations in all formulations at day 0 (date of preparation) and at days 1, 4, 7, 14, 21, and 28. The concentration was compared to a reference standard from the USP.
What they found
While concentrations remained within 90% to 110% of the reference standard for the first week, these levels dropped to < 20% by week 2 and remained at this level throughout (range = 14% to 18%). Storage temperature did not appear to have an effect on the loss of concentration. After seven days of storage, the quality of the formulations also diminished as noted by changes in consistency, visible separation of phases noted in the bottle, and a dark-brown band that appeared near the bottom of the bottle.
Use of compounded doxycycline beyond seven days may not be effective. Veterinarians should be cautious when using compounded medications and be sure to prescribe and dispense these medications in accordance with federal and state regulations.
Papich MG, Davidson GS, Fortier LA. Doxycycline concentration over time after storage in a compounded veterinary preparation. J Am Vet Med Assoc 2013;242(12):1674-1678.
Link to abstract: http://avmajournals.avma.org/doi/abs/10.2460/javma.242.12.1674