Cheristin flea prevention for cats (Eureka)

I have a box of Cheristin for Cats flea protection with 5 topical dose packets left. The strange thing is there is no expiration date. I looked up online and apparently since it is not a medication a expiration date is not required, however, the manufacturer states it has a shelf life of 4-5 years and I purchased mine approximately 1 1/2 yrs ago.