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You can give cranberry extract to cats. It will not get rid of a urinary infection. It is better used as a preventive then a cure. There is not contraindication to give it at the same time as the antibiotics. There are reports about possible calcium oxalate formation. It is believed it helps prevent certain type of bacterial infection. I like to put my recurrent urinary tract infection kitties and dogs on it.
A note about urinary tract problem in cats (FLUTD which means Feline Lower Urinary tract Disease) , 5% of those are caused by a urinary infection. Most of us do put cats on antibiotics the first time around.
I recommend a urine analysis, sometimes we do a culture of the bacteria, and depending of the case some radiographs and/or ultrasound.
The fact he still is having problem urinating may tell us he may have something else going on: crystals/stones/idiopathic cystitis/other.
Here is a link about FLUTD:
I would recommend another visit to your primary care veterinarian. If he strains to urinate and nothing is coming out, it becomes urgent to seek veterinary care.
Let me know if you need additional information.
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