My cat recently had urinary crystals. I've been told cranberry capsules are good for helping clear them up.
Is there any evidence to support this?
The short answer is no.
It's a commonly enough held belief but there's very little evidence to support this being the case.
Cranberry supplementation can help prevent certain Urinary Tract Infections (UTIs) by preventing the bacteria from adhering to the bladder walls (the linked study is in dogs but the same mechanism works in cats, and to a certain extent in humans as well) - but this only seems to apply for certain strains of E.Coli, and while the evidence so far is encouraging in this it's still not exactly concrete.
What's more because cranberry contains high levels of oxalate, which is detrimental for urinary conditions where the cat or dog has oxalate crystals/stones in the bladder (there's some evidence to suggest Burmese, Persian and Himalyan cats are genetically predisposed to Oxalate crystal formation). So well meaning attempts to use cranberry as a treatment for such crystals can actually make the condition worse.
The best method for helping with preventing urinary crystals in cats (both in terms of reducing their impact and formation) is a good diet and plenty of water.
For treating them the golden rule applies - take kitty to the vet and follow the appropriate treatment they recommend. There can be various reasons for them forming and only a vet is going to be able to tell you which - and the treatment will be determined by the root cause.
Feline Lower Urinary Tract Disease (to give urinary crystals their swanky name) can be a serious issue for cats - especially males who have narrower urinary tracts and are less able to "pass" the stones themselves.