Lydia Post Pill is an emergency contraceptive pill taken after unprotected sex or contraceptive failure to prevent unintended pregnancies. It contains hormones that inhibit ovulation and fertilization. It should be taken within 72 hours but can be effective up to 120 hours afterward. Side effects may include nausea and irregular bleeding. It does not protect against STIs and should not replace safe sex practices. Consult a healthcare professional before use.