Can Plan B Alter Menstrual Cycle Timing?
Yes, taking Plan B (levonorgestrel) 3 weeks ago can definitely change her menstrual schedule, and this is an expected, normal side effect that typically resolves within one cycle.
Expected Menstrual Changes After Plan B
The FDA labeling for levonorgestrel explicitly warns that "some women may have changes in their period, such as a period that is heavier or lighter or a period that is early or late" 1. The timing and nature of these changes depend critically on when in her cycle she took the medication:
If Taken Early in the Cycle (Before Ovulation)
- The current cycle will likely be shortened - her period may come earlier than expected 2, 3
- The earlier in the cycle Plan B was taken, the more pronounced this shortening effect 2
- In one study, women who took levonorgestrel 3 days before expected ovulation had their cycles significantly delayed (mean delay of 16.8 days in some cases) 4
- Another study showed cycles could be delayed from 26.4 to 39.1 days when taken before ovulation 3
If Taken Mid-Cycle (Around Ovulation)
- The cycle may be significantly shortened - potentially by 5 days or more 3
- One study found cycles shortened from 25.1 to 20.2 days when taken just before ovulation 3
- The mechanism involves disrupting corpus luteum formation and causing premature luteinization 3
If Taken Late in the Cycle (After Ovulation)
- Changes are generally less dramatic but still possible 2
- The duration of menstrual bleeding itself may be longer than usual 2
Additional Menstrual Pattern Changes
Beyond timing, she may experience:
- Heavier or lighter bleeding than usual 1, 5
- Changes in menstrual appearance (texture, color) 5
- Intermenstrual bleeding (spotting between periods), though this occurs in only about 5-15% of women 5, 2
- Longer duration of the actual menstrual period 2
Critical Clinical Action Point
If her period is more than 1 week late from when expected, she needs a pregnancy test 1, 6. The CDC guidelines specifically recommend advising patients to "have a pregnancy test if they do not have a withdrawal bleed within 3 weeks" 6.
Reassurance About Resolution
These menstrual changes are transient and typically resolve by the following cycle 5. The majority of menstrual disruptions disappear after the first post-treatment cycle 5.
Important Safety Caveat
If she develops severe abdominal pain, this could indicate an ectopic pregnancy and requires immediate medical attention 1. While Plan B is highly effective, it does not prevent 100% of pregnancies, and when it fails, the risk of ectopic pregnancy must be considered 1.