Flea dates punish vague memory faster than yearly dates
Annual checkups drift by weeks without much harm, but flea prevention runs on a repeating cycle where the errors are specific: one cat treated twice in a short span because two people each thought the other had not, another cat quietly skipped because she hides at treatment time, a gap long enough for fleas to re-establish. A filing drawer folder split by cat attacks the mix-up problem directly, because each animal's applications are recorded in her own section and nowhere else. The folder must be arranged for fast, dated entries, though, not just for storing packaging and receipts.
Put a running log page at the front of each section
The first page of every cat's section should be a plain running log ruled into columns: date applied, who applied it, and next due date. Each treatment adds one line, written the same day, before the empty packaging is thrown away. Writing the next due date at the moment of application is the crucial move, because it converts the manufacturer's interval on the packet you are holding into a calendar fact in the cat's own record, instead of a sum somebody must do later from memory. Keep the log page clipped to the front divider so opening the section means seeing the last line first.
File the supporting paper behind the log, newest first
Behind the log, file anything that substantiates it newest-first: receipts showing what was bought and when, the outer packaging sleeve if you keep it for reference, and any vet note about a skin reaction or a change of product for that cat. This order matters during the two moments the folder gets used in a hurry: confirming whether a cat was genuinely treated before a suspected double application, and answering a vet who asks what has been used on this cat and since when. Resist keeping one combined flea file for the household; combined files are exactly how individual cats' histories blur.
Reconcile the folder against the cats monthly
A drawer log stays true only if someone compares it with reality on a rhythm shorter than the treatment cycle. Choose a fixed monthly moment, such as the day household supplies are ordered, and read down every cat's log: who is due this week, whose next due date has passed with no new line, and whether the number of treatments logged matches the number of doses actually used from the box. Discrepancies surface while they are still small. If a cat was treated away from home, by a sitter or at the vet, add that line retrospectively with a note of who confirmed it, rather than leaving a gap the next person will misread as a missed treatment.
When a reaction or a vet question outruns the drawer
The drawer holds the truth, but it holds it at home. If a cat scratches frantically, develops sore skin after an application, or seems unwell, the questions come at the surgery or on the phone: what was applied, on which date, and what happened last time. Photographing log pages helps in a pinch, yet a maintained copy travels better and can be shared with the family member actually holding the cat. For the appointments where those dates get questioned, KinsLe Pet Passport puts this cat's record in the carrier with her, so answers travel instead of waiting in a drawer at home.