Plan B half-life and pharmacokinetics
Half-life describes how long it takes plasma concentration of Levonorgestrel to drop by half after a dose. It is the most useful single number for understanding why Plan B (Levonorgestrel) is dosed the way it is — once daily, on demand, or some other schedule. The 1.5mg strengths and tablet formulation tune the curve.
Why half-life matters
Levonorgestrel reaches peak plasma levels some hours after dosing, then decays. Short half-life agents are out of the system quickly and well-suited to event-driven dosing. Long half-life agents allow once-daily continuous coverage but accumulate over the first few days until reaching steady state. Plan B's primary mechanism is delaying or preventing ovulation when taken before the LH surge.
Practical dose-pacing
According to the prescribing information for Levonorgestrel, the dosing interval reflects the half-life and the desired duration of effect. Re-dosing inside the half-life window stacks plasma concentration without proportional benefit; spacing doses correctly keeps the steady-state where it is expected. The 1.5mg options exist to allow personalised exposure within this framework.
Frequently asked questions
How long does Plan B stay in your system? ▾
Most active drug clears within four to five half-lives. For Levonorgestrel the exact half-life is in the prescribing information, but residual measurable drug may persist longer than the subjective effect at 1.5mg.
Can Plan B accumulate over time? ▾
Daily dosing of any drug accumulates until plasma concentrations reach steady state, typically within four to five half-lives. After that, Plan B stays at predictable levels as long as the 1.5mg dose is unchanged. This is by design and is not the same as harmful accumulation.
More on Plan B
- With alcoholPlan B and alcohol — is it safe to drink?
- With foodShould Plan B be taken with food?
- Side effectsPlan B side effects: common, rare and warning signs
- Dosage guidePlan B dosage guide: how much to take and when
- OnsetHow fast does Plan B start working?
- DurationHow long does Plan B last?
The information on this website is provided for reference and educational purposes only. It does not replace consultation with a qualified healthcare professional.