Researchers at Kansas State University found that formulations of doxycycline from compounding pharmacies can lose potency within 3 weeks, falling below therapeutic levels within that time. Chews and liquid formulations were more likely to be affected than tablets. FDA-approved formulations that were tested, on the other hand, all maintained potency over the course of the study.

Read more about the study of compounded drug loss of potency in Veterinary Practice News. The original research can be viewed in JAVMA. 

More information on compounded drugs and usage of tetraycyclines can be found in the MSD Veterinary Manual.