No worries there Jo, it is always better to give an overdose in milligrams then grams (since its a smaller dose). Now as you appreciate this dose is larger compared to the standard dose we use in cats (~1mg/kg). Still based on what she may have had (25-50 mg)and if we said she was the average sized cat (ie 4 kg), then we can work out her toxicity risk and potentially for adverse effects.
For a 4kg cat, this would be an ingestion dose range of 6.25- 12.5 mg/kg. And the good news here, is that this drug is actually quite well tolerated. In fact, studies have shown that overdoses up to 50 mg/kg in cats were asymptomatic and caused no adverse signs. And what she has potentially had here was only a quarter to an eighth of that. Therefore, her risk of any adverse signs are small.
Just to note the kinds of adverse effects we can see with nitenpyram, the main ones we see are weakness, depression, difficulty breathing, GI upset (ie drooling, appetite loss, vomiting, diarrhea), wobbliness, cramps (muscle twitching), tremors, and seizures. But again with such a small overdose, we would likely see the drooling and GI upset if anything.
Therefore,in this case, we do have a few options. You could choose to induce vomiting (How To), administer activated charcoal from the chemist to block absorption (More Info), or you could have your neighbor monitor Polly's appetite. If you do not think they are capable of doing so or you want to err on the side of caution, do consider asking your vet to hospitalize her for observation for 48 hours (the time frame it takes for this drug to leave the system) to make sure she has no issue while you are away. After that time frame, they could keep her in for you or release her to your neighbor.
Otherwise, we'd want your neighbor to monitor her appetite and perhaps offer a light diet if she doesn't seem keen to eat. Examples of light diet options are boiled chicken, white fish, scrambled egg, or meat baby food -- as long as its free from garlic or onion powder. As well, there are veterinary diets that are good for tummy upset (ie Royal Canin Sensitivity Control, Hill's I/D, etc). These could be fed over the next few days if there is any worry of stomach upset. Otherwise, if they think she is being fussy over food due to this wee overdoses' adverse effects, then we'd want them to have her to the vet to make sure she doesn't just starve herself while you are away.
Finally, just to note since you are on a time crunch, most veterinary practices here do have contingency plans for emergency care for their patients even when they are not open. Therefore, if you wanted to have her checked tonight, wanted them to induce vomiting, or just admit her for you, it is worth ringing the practice. If they are open, you can sort that now. If they aren't, then they will likely have a message to direct you on how to contact their out of hours service. And if you don't have a vet you can find one local to you, you can check the RCVS Register (HERE) to find your local vets or Vets Now (LINK) who are open all nights/weekends.
Overall, this is a large dose for her but it is a drug that is well tolerated. So, you can take the above intervention steps to limit any signs but if you do not see any GI upset signs over the next 15 hours, then it is likely you will be in the clear for this wee overdose and likely can just have your neighbor monitor her while you are away.
I hope this information is helpful.
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All the best,
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