Polyprenyl Immunostimulant is a very safe product which has been tested and/or used by now in about 1000 cats in 29 countries around the world with no reports of adverse events. Safety studies were conducted in 390 cats located in 10 states, from single- and multiple-cat environments. The cats belonged to private owners, rescuers, breeders, or resided in sanctuaries and shelters. The cats' ages ranged from 2 days up to 17 years, and about half of all cats were pedigreed and belonged to 7 registered breeds.
The study showed that oral application of 0.5 mg/kg Polyprenyl Immunostimulant twice daily for 15 days is safe for cats at or above 8 weeks of age. No adverse events were reported, albeit cats may not like the taste, or the smell. How many people expect their cat to love being medicated, in overall? We know of two who craved PI which should be due to the unpredictability of cats, as we know them. No adverse events were noted in the test when five cats were given 10x of the therapeutic dose (5 mg/kg) of Polyprenyl Immunostimulant. In tests with mice, 100x therapeutic doses are used, and those cause no adverse effects either.
Veterinarians also study Polyprenyl Immunostimulant in other infections and other species, like dogs, ferrets, and birds.
You may read about FIP research by Dr. Alfred Legendre in his paper and on his site http://www.vet.utk.edu/research/fip/fip.php.