How long does plan b stay in your blood?

The active ingredient in Plan B, levonorgestrel, can be detected in the blood for up to 3 days after taking the medication. However, the amount of levonorgestrel in the blood decreases rapidly over time. By 24 hours after taking Plan B, the concentration of levonorgestrel in the blood is about half of what it was immediately after taking the medication. By 48 hours after taking Plan B, the concentration of levonorgestrel in the blood is about one-fourth of what it was immediately after taking the medication.