Plan B Does Not Cause Weight Gain
Plan B (levonorgestrel emergency contraception) does not cause weight gain. This is a single-dose emergency contraceptive, not a continuous hormonal method, and the available evidence shows no association between levonorgestrel emergency contraception and weight changes.
Understanding Emergency Contraception vs. Continuous Hormonal Methods
The confusion about Plan B and weight gain likely stems from misunderstanding the difference between emergency contraception and ongoing hormonal contraceptive methods:
- Plan B is a one-time dose of 1.5 mg levonorgestrel taken after unprotected intercourse 1
- The medication is cleared from the body within days, providing no sustained hormonal exposure that could affect weight 1
- Weight gain concerns documented in contraceptive literature relate to continuous use methods like depot medroxyprogesterone acetate (DMPA), implants, or daily oral contraceptives—not emergency contraception 2, 3
Evidence on Levonorgestrel and Weight
Studies examining continuous low-dose levonorgestrel contraceptives (which provide far more cumulative hormone exposure than a single Plan B dose) show:
- Oral contraceptives containing levonorgestrel (20-30 μg ethinyl estradiol plus 100-150 μg levonorgestrel taken daily) cause no clinically or statistically significant weight changes over 3-4 months in either normal weight or obese women 4
- A study of 47 women using daily levonorgestrel/ethinyl estradiol showed no impact on body weight, fat mass, fat-free mass, or body water composition 5
- Levonorgestrel intrauterine systems (which release levonorgestrel continuously for years) show minimal weight change, with adjusted analyses finding no significant difference compared to non-hormonal copper IUDs 6
Why Plan B Cannot Cause Weight Gain
The pharmacology makes weight gain from Plan B biologically implausible:
- Single exposure: One 1.5 mg dose provides brief hormonal exposure measured in hours to days, not the weeks to months required for metabolic changes affecting weight 1
- Dose comparison: Even continuous daily levonorgestrel contraceptives (providing cumulative doses far exceeding Plan B over time) don't cause weight gain 4, 5
- No fluid retention mechanism: The transient hormonal spike from Plan B resolves too quickly to cause sustained fluid retention or metabolic changes 1
Common Pitfall to Avoid
Do not confuse emergency contraception with continuous contraceptive methods. The weight gain literature cited for hormonal contraceptives applies to DMPA injections, implants, and daily pills used continuously—not single-dose emergency contraception 2, 3. Women concerned about weight should be reassured that Plan B poses no weight gain risk, though they should understand that levonorgestrel EC loses effectiveness in women weighing more than 165 pounds and becomes ineffective above 176 pounds 1.
Clinical Counseling Point
If a patient reports weight gain after taking Plan B, this represents either:
- Normal weight fluctuation unrelated to the medication 4
- Coincidental timing with other life factors affecting weight 6
- Perception bias from anxiety about the medication 7
The American Academy of Pediatrics confirms that weight gain has not been reliably linked to combined hormonal contraception, and this applies even more strongly to single-dose emergency contraception 3.