there are a few things, piriton is a really common one
but they even give dogs viagra for heart conditions.
our vet told us to give our dog half a paracetamol tablet when her hips were causing her trouble
(meaning a 250mg dose)
we used to get her regular courses of an antihistamine from the vet when she was younger as she had a few problems when the seasons changed, as these episodes calmed down they said a piriton tablet every now and then if she was having trouble would be okay
in fact one of the last meds she was on towards the end of her life was a human medication - they were like horse tablets, terrible to get down her, considering she was a 17kg border collie and had a tumor in her throat at the time.
they were to prevent her being sick, the vet basically told us they're such large tablets as it's a fair bit cheaper to get the human pills, than the animal use ones.
fungal treatments like daktarin can be used on dogs or guinea pigs & rabbits to treat small areas of ringworm
i always keep a tube in in case of any flaky guinea ears.
i went into a boots once and asked if they had any generic (cheaper branded) miconozole nitrate cream to use on one of the guinea pigs, they thought it a bit strange that i was asking for it for that use, and to be honest it's a fair point on their side, they're not really there to know or advise on animal treatments.
i took in printed research to back up my claims that it was safe though.