I'm Dr. Jo and I'm here to help you with your question about Fang. I'm so sorry this has happened, but glad you're looking for the information you need.
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As I'm sure you're accustomed to, pretty much everything regarding the use of any drug in a rabbit is at best an educated guess based on what we know about how drugs work in other species. Unfortunately, we have absolutely NO good studies on the usage or dosing of cyproheptadine in rabbits period, let alone overdose information. Because of this, we don't know if it even works for rabbits, what dose to use, and if it's safe under normal circumstances. That means it's impossible to know for certain how Fang is likely to respond to this doubled dose. In most cases, we would expect no problems, but cyproheptadine *does* have some side effects, and these can be potentiated in the case of a doubled dose like Fang received.
Please don't misunderstand. This doesn't mean your vet has done something wrong by prescribing it. In situations like this, we have no option other than to make our best guess and go with it.
Here is what Plumb's formulary has to say about cyproheptadine overdoses (regardless of species):
There are no specific antidotes available. Significant overdoses should be handled using standard gut emptying protocols when appropriate and supportive therapy when required. The adverse effects seen with overdoses are an extension of the drug’s side effects, principally CNS depression (although CNS stimulation may be seen), anticholinergic effects (severe drying of mucous membranes, tachycardia, urinary retention, hyperthermia, etc.) and possibly hypotension. Physostigmine may be considered to treat serious CNS anticholinergic effects, and diazepam employed to treat seizures
, if necessary.
Horses that have received doses 2X greater than recommended apparently showed no untoward effects."
And here is the listing of side effects:
Cyproheptadine is contraindicated in patients hypersensitive to it. It should be used with caution in patients with prostatic hypertrophy, bladder neck obstruction, severe cardiac failure, epilepsy
, angle-closure glaucoma, or pyeloduodenal obstruction.
The most likely adverse effects seen with cyproheptadine are related to its CNS depressant (sedation) and anticholinergic effects (dryness of mucous membranes, etc.). Cats can develop a paradoxical agitated state that resolves upon dose reduction or discontinuation. There have been reports of cyproheptadine-induced hemolytic anemia in cats. Horses may show mild depression, anorexia, or lethargy.
At higher dosages, cyproheptadine has caused significant polyphagia in dogs."
Because Fang has a cataract, she is at greater risk for developing glaucoma. This doubled dose could increase those odds even further. That's the primary adverse effect I'd be worried about in your case.
All things considered, however, the drug has a wide margin of safety and it's most likely she won't experience any bad side effects from the doubled dose.