Please help! do i need plan b? sperm was inserted in vagina on day 16 of menstrual cycle. it has already been 24 hours! help!

See answer: Day 16 may well be within your fertile window. If want to minimize/eliminate the chance of pregnancy, then "emergency contraception" with "morning-after-pill" or Plan B recommended. The sooner you take (and definitely no longer than 5 days post-intercourse), the greater the effectiveness.

Agree w Dr. Lewis: If you were not using any form of protection & you don't wish to become pregnant- Then yes, an emergency contraceptive is recommended. Plan B One-Step is up to 89 % effective when taken within 72 hours (3 days) after unprotected sex. It can reduce chance of pregnancy by up to 95% if used within 24 hours. It continues to reduce the risk of pregnancy up to 120 hours (5 days) after unprotected sex,