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Good Morning and welcome to Just answer. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
I would worry about any color change in this medication. This will indicate that the medication has lost its efficacy but does not normally cause any toxicity. the best thing for you to do now is find the manufacturer of this product and call their help line. They should have a doctor well versed in this medication. The package insert also will give you information on this medication.
In my 30 years as a vet, I have never had a cat that had become ill from a medication that had not been refrigerated or that had turned a different color. That is not to say that this did not happen...it is just not in my experience.
Cal the manufacturer of this Doxycycline to know for sure....
Sincerely, Dr. Loretta