## **Core Concept**
The question revolves around the use of **levonorgestrel**, a form of emergency contraception, specifically its dosing regimen when taken after unprotected intercourse. Levonorgestrel is a synthetic progestogen that can prevent pregnancy when used as emergency contraception.

## **Why the Correct Answer is Right**
The standard regimen for **levonorgestrel** as emergency contraception is 0.75 mg taken twice, 12 hours apart. If the first dose of 0.75 mg has already been taken, the next dose should be administered **12 hours later**. This specific regimen is designed to effectively prevent ovulation or interfere with a fertilized egg's implantation in the uterus, thereby preventing pregnancy.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests taking the next dose immediately, which does not align with the recommended 12-hour interval between doses.
- **Option B:** This option suggests waiting 24 hours, which would extend the interval beyond the recommended 12 hours and potentially reduce the efficacy of the emergency contraception.
- **Option D:** This option suggests waiting 6 hours, which is too short an interval and does not comply with the standard dosing regimen.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that emergency contraception like **levonorgestrel** is most effective when used as soon as possible after unprotected intercourse, ideally within 72 hours. The 12-hour dosing interval for levonorgestrel is crucial for its effectiveness, and understanding this regimen helps in providing optimal care.

## **Correct Answer:** B. 12 hours later.