Sentry PurrScriptions Plus Flea Collar for Cats over 12 weeks of age kills fleas and flea eggs for up to 6 months. Collar is a sustained-release system containing four active agents working to Provide head-to-tail Protection. Thus, collar has the effect of simultaneous usage of a spray and powder in the control of fleas. Use Sentry PurrScriptions Plus Flea Collar for Cats over 12 weeks of age year round for most effective control. The adult fleas you see on your pet make up only part of an infestation; over 90% of a flea infestation can lie in the pre-adult forms of the flea egg, larvae, pupae. This collar Protects your pet against reinfestation for up to 6 months. To Protect against adult fleas, the collar should be replaced every 6 months or sooner if needed.