Nexplanon starts working immediately if it is inserted within the first five days of the menstrual cycle. If it is inserted at any other time during the cycle, backup contraception is required for seven days to ensure full protection from pregnancy. The implant works by releasing the hormone etonogestrel, which prevents ovulation, thickens cervical mucus, and alters the uterine lining to prevent pregnancy. If switching from another hormonal contraceptive or after childbirth or miscarriage, the timing for immediate protection can vary, with specific guidelines to follow to avoid pregnancy risk. Consulting a healthcare provider ensures the best timing and coverage.