I have had occasion to use both oral and implant melatonin.  I do not
know what your ferret symptoms are but I found the response to be quick.
Almost immediately fur will start to regrow and the appetite increases
(a side effect is indeed a great deal of weight gain) as well as a very
pleasant personality returning.  I have had great success with melatonin.
I suspect if it is going to work for your ferret you will notice these
signs very soon.  I hope all the best for you it is heartbreaking for
us ferret moms when our fuzzbutts are suffering.
 
PS-they will sleep more in the beginning but that seems to let up over
time as they are on melatonin and they start to play again.
 
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[Posted in FML issue 5258]