What is Desogestrel and How Does It Work?
Desogestrel is a synthetic hormone, a type of progestogen, used in progestogen-only contraceptive pills (POPs), often called the "mini-pill" [1.6.5, 1.10.4]. Unlike combined oral contraceptives that contain both estrogen and progestogen, desogestrel pills only contain this one type of hormone [1.8.1]. This makes them a suitable option for individuals who cannot take estrogen, such as those over 35 who smoke, have certain types of migraines, or are breastfeeding [1.8.4, 1.10.3].
The primary mechanism of action for desogestrel is the prevention of ovulation, meaning it consistently stops the ovaries from releasing an egg [1.6.4, 1.10.2]. In addition to inhibiting ovulation, desogestrel also works by:
- Thickening cervical mucus: This creates a barrier that makes it difficult for sperm to travel through the cervix and reach an egg [1.10.2].
- Thinning the uterine lining (endometrium): This makes the lining less receptive to a fertilized egg, preventing implantation [1.7.1, 1.10.2].
This dual action makes desogestrel a highly effective contraceptive method when taken correctly, with a failure rate of less than 1% with perfect use [1.3.2, 1.7.1].
Does Desogestrel Work Straight Away?
The answer depends entirely on when you start taking the first pill in relation to your menstrual cycle.
- Starting on Days 1-5 of Your Period: If you take your first desogestrel pill within the first five days of your menstrual cycle (with day one being the first day of your period), you are protected against pregnancy immediately. You will not need to use an additional form of contraception [1.3.3, 1.4.1, 1.10.5].
- Starting at Any Other Time: If you start taking desogestrel on any other day of your cycle, you will not be protected straight away. It takes time for the pill to become effective. You must use a barrier method of contraception, such as condoms, for the first two days (48 hours) of taking the pill to prevent pregnancy [1.3.3, 1.4.1].
Special Circumstances for Starting Desogestrel
- After Childbirth: You can start taking desogestrel between day 1 and day 21 after giving birth, and you will have immediate contraceptive protection [1.3.3]. If you start more than 21 days after giving birth, you need to use extra contraception for the first two days [1.3.3]. Desogestrel is considered safe to use while breastfeeding as only small amounts pass into breast milk and it does not affect milk production [1.2.5, 1.10.1].
- After a Miscarriage or Abortion: You can start desogestrel up to five days after a miscarriage or abortion and be protected straight away [1.3.3, 1.4.1]. If you start more than five days later, you must use backup contraception for the first 48 hours [1.3.3].
- Switching from Another Contraceptive: If you are switching from a combined pill or another POP, the rules for when protection begins can vary. It's crucial to consult the patient information leaflet or a healthcare provider for specific instructions to ensure continuous coverage [1.4.4].
Comparison: Desogestrel POP vs. Combined Pill
| Feature | Desogestrel (Progestogen-Only Pill) | Combined Oral Contraceptive (COC) |
|---|---|---|
| Hormones | Progestogen only [1.10.2] | Estrogen and Progestogen [1.8.1] |
| How it Works | Primarily stops ovulation, thickens cervical mucus [1.6.4] | Stops ovulation, thickens cervical mucus, thins uterine lining [1.6.3] |
| Usage | One pill taken daily, no break between packs [1.3.2] | Typically 21 daily pills followed by a 7-day break or placebo pills [1.3.5] |
| Missed Pill Window | 12 hours for desogestrel pills [1.5.5] | Varies, but generally less strict than traditional POPs [1.4.3] |
| Suitability | Suitable for many who can't take estrogen (e.g., smokers over 35, breastfeeding) [1.8.4] | Contraindicated for people with certain health risks (e.g., history of blood clots, certain migraines) [1.8.4, 1.8.5] |
| Effect on Periods | Can be irregular, lighter, or stop altogether [1.3.5] | Periods are usually regular, lighter, and less painful during the break week [1.8.1] |
What Can Reduce Desogestrel's Effectiveness?
To ensure desogestrel works correctly, you must take it at the same time every day. Desogestrel-based pills allow for a 12-hour window, meaning you are still protected if you take your pill within 12 hours of your usual time [1.5.5, 1.10.3]. If you are more than 12 hours late, its effectiveness is reduced.
Missed Pill Rule (More than 12 hours late):
- Take the missed pill as soon as you remember, even if it means taking two in one day [1.5.3].
- Continue taking the rest of the pack as usual at your normal time [1.5.3].
- Use a barrier method of contraception (e.g., condoms) for the next 2 days [1.5.3].
Other factors can also reduce effectiveness:
- Vomiting or Severe Diarrhea: If you vomit within 3-4 hours of taking your pill, or have severe diarrhea, the pill may not have been fully absorbed [1.5.4]. You should treat this as a missed pill and use additional contraception for 2 days after you recover [1.5.2, 1.5.4].
- Interacting Medications: Certain medicines can interfere with how desogestrel works, making it less effective. These include some treatments for epilepsy, HIV, tuberculosis, and the herbal remedy St. John's Wort [1.9.1, 1.9.5]. Always inform your doctor or pharmacist of all medications you are taking.
Conclusion
So, does desogestrel work straight away? The answer is a conditional yes. It provides immediate protection only if you start it within the first five days of your period. At any other time, its contraceptive effect is delayed, requiring backup protection for the first 48 hours. Consistent daily use at the same time, and awareness of factors like missed pills and interacting medications, are essential for maintaining its high efficacy rate of over 99% with perfect use [1.10.3].
For authoritative information, you can visit the NHS page on the progestogen-only pill.
