Depends on the version....if it is a compounded drug, refrigeration is to keep it stable. I probably wouldn't use any drug thatshould have been refrigerated but wasn't. But I have other concerns. First, rabbits don't get "kennel cough". They can be carriers of bordetella but rarely present active symptoms. Doxycycline is in the family of Tetracycline. In all my years of treating rabbits, I have only used it once and that was in treatment of a streptoccocus infection. Without a proper culture, use of antibiotics is a guessing game. Most of the time, what many vets may call "kennel cough" is actually an Upper Respiratory Infection that is usually caused by a bacteria knonw as Pasteurella Multocida. The traditional drug used has been Baytril but the bacteria is becoming resistant and that drug doesn't work so well these days. My personal drug of choice against a confirmed Pasteurella infection is Zithromax. I have also used Zeniquin (a new floroquinolone related to Baytril but without the resistance issues) or one of the injectable antibiotics such as Convenia or Penicillin. My non-professional opinion is that Doxycycline isn't the best choice in this situation.
Randy