What is the mechanism of action of levonorgestrel when used as "Plan B"?

Levonorgestrel, when used as "Plan B," primarily acts as a progestin. Its mechanism of action involves preventing or delaying ovulation. By doing so, it inhibits the release of eggs from the ovaries. Additionally, it alters the endometrial lining, which may make it less conducive for implantation should fertilization occur.

In contrast to other methods, such as combined oral contraceptives that typically contain both estrogen and progestin, levonorgestrel is specifically a progestin-only regimen. This can be particularly advantageous for individuals who may have contraindications to estrogen.

The other options are less relevant to the specific mechanism of levonorgestrel. For example, estrogen receptor antagonists do not fit the description of levonorgestrel's action, as it does not function by blocking estrogen receptors. A combined oral contraceptive includes both estrogen and progestin, which is different from the progestin-only nature of levonorgestrel. Similarly, a luteinizing hormone blocker does not accurately represent its action, as levonorgestrel's primary effect is not on luteinizing hormone but rather on ovulation prevention through progestin activity.