My cat got too much medicine. Is an accidental overdose serious?

According to the veterinary medicine resource this drug has been tolerated at doses up to 90 mg/kg with mild symptoms (anorexia and excessive drinking) in some cats. Callie would have to have eaten 450 mg of Famciclovir to see these symptoms, so she is well below this amount if she ingested 2-125mg tablets totaling 250mg. She should be fine, however you can also consult with pet poison helpline to talk it over with a certified toxicologist as well. But based on my minimal research she is below the toxic level for ingestion. I hope your sweet girl continues to do well. Take care.

Unfortunately, this could be serious. As I'm sure you know, this is an extended release product, and the typical dose is 100mg per cat every 24 hours, so yes, I believe you very well might see toxicity from this, including (on the mild side) excitement, hyperactivity, vomiting, and diarrhea, all the way up to seizures. You have a couple of options: you can call the ASPCA's toxin hotline at (888) 426-4435; they will charge you for the call ($45) but they keep an extensive database on all types of overdoses and will be able to tell you if this is a significant overdose. Your other option is to take Nox to an emergency clinic now to have vomiting induced, and have him treated for the ingestion. They will be able to administer activated charcoal to absorb any residual toxins in the gut as well.

I would not be overly concerned, although the recommended dose is 5-10mg/lb (and an 8lb cat would have consumed approx 15mg/lb) it does have a wide safety margin and overdose symptoms are very rare. I would expect some mild gastrointestinal upset and a probiotic would be helpful to reduce the impact on the natural gut balance

This should be fine as most commonly used antibiotics have wide dose ranges and good safety margins. It does depend on the specific antibiotic though and dosage, to be safe you can either ring your vet for advice or repost with more details here.

She will be fine. Dewormer has a really wide safety margin so double dosing does not cause any major issues for pets. At the most she may have a little upset stomach, but I don't even expect that to occur.

With your cat's weight being 11 pounds, there is a dose range for Buprenorphine that fits into the amount you gave. He may be a little sleepier and I would not continue the double dose unless your vet recommends it. Keep an eye on him in case he is too sleepy, but he should be ok. Signs of a problem would be too sleepy, a slower breathing rate.

This represents a pretty substantial overdose. Corbett got about 2.5 times more than the highest published dose for pain relief. However, it's thought that buprenorphine has a high index of safety, and even large overdoses don't usually cause serious effects. I'm assuming you gave it on the gums ("transmucousally") as that's how it's typically prescribed for pain relief at home in cats. Because of that method of administration, there's no way to remove the drug from his system by inducing vomiting. There is a reversal agent that is somewhat effective, but may have to be given in multiple doses due to the way buprenorphine works. The drug is naloxone, and your veterinarian should have this drug. Because the signs of overdose can be concerning (vocalization, ataxia, hypersalivation, decreased body temperature, and lethargy) my best recommendation would be for you to take Corbett back to your veterinarian for administration of this drug, so that the overdosage can be averted.