kitten & cat with severe diarrhoea

basically i have a three month & 11 month old kittens (both male, the older neutured) who have both had diarrhoea for atleast a fortnight now.
   
i took my 3 month old kitten to the vets last tuesday & spent £50 on some antibiotics for both cats and something called pro-kolin which is supposed to make the stools harder & i have followed instructions, giving both twice a day to both kittens, although having now finished the antibiotics i am dissapointed & also worried that they aren't any better, they both still have diarrhoea.
   
it's dark brown & very runny, there hasn't been any sickness with it & they both seem themselves.
   
i shall take them back to the vets (even though i can't really afford it) when i can get an appointment, but wondered if anyone had, had something similar happen to their cats & if so how was it resolved?
   
thanks.

Did the vet run any tests on the stool, looking for bacteria or parasites? We had a similar experience with our cats?it turns out that our adopted stray had brought the protozoal parasite Giardia into the house, and the other two cats *and* the dog all contracted it. It's horrible stuff, and hard to kill with regular antibiotics?AND it's transmissable to humans. At the very least, for your own peace of mind, you'll want to rule that out.
   
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Jack
   
http://www.purelydogbeds.com

I think it is the waste of time and money by visiting to vet to many times. I have found the site with a good home tips for dog http://www.petmanage.com/dog-health/dog-digestive-problems.html