Oh dear, she is quite a small lass.
Therefore, this is an overdose if she did have it and we could see her develop GI upset or a stomach ulcer. So, we are best being proactive for Lily. Ideally, we'd want to induce vomiting to get as much of this out of her stomach as possible. To do that, you can use washing soda crystals (NOT CAUSTIC SODA!). Take a small handful of the soda crystals, mix with a bit of water to make a ball and give it right down her throat. If you do that and she doesn't bring this back up, then you'd need to consider seeing your local vet (or ER vet) so that apomorphine (a very strong injectable emetic) can be administered just get this out of the stomach and avoid any adverse issues.
Else we could (and ideal if you aren't sure if she had it but think possibly not) consider administering activated charcoal at this stage. This is available over the counter from the pharmacy (ask for the high strength version, not the one for gas) and works by binding any remaining material in the stomach. For activated charcoal, we tend to give 1-4 grams per pound every 8 hrs. This can be mixed with food to be fed or with water to syringe feed (do note that it stains, so keep it away from white carpets/clothes). This will just limit how much is absorbed and reduce the intoxication risk here.
Overall it is a bid dose if she had this, so we need to tread with care and err on the side of caution by using the above. If you struggle to do this at all, then we need her seen and you will need to ring her vet (as all have out of hours care options) or check the RCVS Register (http://findavet.rcvs.org.uk/find-a-vet/) or Vets Now (http://www.vets-now.com/find-an-emergency-vet/) who are open all nights/weekends.
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