The LNG-releasing intrauterine system is designed as a long-acting, locally acting contraceptive with a defined period of reliable use. It steadily releases levonorgestrel for about five years, long enough to consistently thin the endometrium and thicken cervical mucus, creating an environment hostile to fertilization and implantation. Because the hormone release tapers over time, efficacy is considered reliable for up to five years; after that, the amount released may drop below the level needed for effective contraception, so removal or replacement is recommended if continued contraception is desired. Shorter durations, such as three years, would miss the labeled window for most LNG-IUS products, while longer durations (seven or ten years) exceed the typical labeling unless a specific device is approved for that extended use.