Mercilon

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Mercilon on Prescriptsy

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Mercilon is a low-dose combined oral contraceptive pill (COC) used to prevent pregnancy.

Each tablet contains two active ingredients: desogestrel 150 micrograms (a progestogen) and ethinylestradiol 20 micrograms (a synthetic oestrogen).

Mercilon is identical to Marvelon in its progestogen content but contains a lower dose of oestrogen (20 micrograms compared with 30 micrograms in Marvelon).

This lower oestrogen content may reduce the incidence of oestrogen-related side effects while maintaining the same high level of contraceptive efficacy.

Mercilon is manufactured by Organon and has been available in the UK for many years.

It is a prescription-only medicine (POM) and is available free of charge through the NHS.

The choice of combined oral contraceptive is influenced by several factors, including the dose and type of oestrogen and progestogen, individual side effect profile, cardiovascular risk, and personal preference.

Low-dose pills like Mercilon are often considered for women who experience side effects on standard-dose (30 microgram ethinylestradiol) pills, including headache, breast tenderness, nausea, and mood changes.

This page provides a comprehensive clinical overview of Mercilon, covering how it works, the clinical rationale for low-dose oestrogen pills, dosage guidance, side effects, important safety warnings, and how to obtain a prescription in the United Kingdom.

Important safety information about Mercilon

Before reading further, please note these essential safety points.

• Mercilon is a prescription-only medicine for pregnancy prevention.
• All combined pills carry a small increased risk of blood clots (VTE). The risk with desogestrel-containing pills is slightly higher than with levonorgestrel-containing pills.
• Do not use Mercilon if you experience migraine with aura.
• Smoking while taking the pill increases cardiovascular risk. Women over 35 who smoke must not use combined pills.
• Because Mercilon contains only 20 micrograms of ethinylestradiol, consistent daily pill-taking is particularly important.

Understanding low-dose combined oral contraception

The term "low-dose" in the context of combined oral contraceptives refers to the amount of ethinylestradiol.

Standard-dose pills contain 30 to 35 micrograms of ethinylestradiol, while low-dose pills contain 20 micrograms. Ultra-low-dose pills containing 15 micrograms are also available in some formulations.

The rationale for reducing the oestrogen dose is to minimise oestrogen-dependent side effects and risks while maintaining reliable suppression of ovulation and effective contraception.

Oestrogen-related side effects include headache, breast tenderness, nausea, bloating, fluid retention, and mood changes.

Some of these effects are dose-dependent, meaning they are less likely to occur at lower oestrogen doses.

However, reducing the oestrogen dose can increase the rate of breakthrough bleeding, particularly in the first few cycles, because a lower oestrogen dose provides less endometrial stabilisation.

The clinical challenge is finding the oestrogen dose that balances effective cycle control with minimal side effects for each individual woman.

The effect of oestrogen dose on VTE risk has been studied extensively.

While there is a biological rationale for expecting lower VTE risk with lower oestrogen doses (oestrogen promotes hepatic synthesis of clotting factors), epidemiological evidence has not consistently shown a significant difference in VTE rates between 20-microgram and 30-microgram pills containing the same progestogen.

The type of progestogen remains the dominant determinant of VTE risk, with third-generation progestogens (desogestrel, gestodene) conferring a higher risk than second-generation progestogens (levonorgestrel) regardless of oestrogen dose.

How Mercilon works: mechanism of action

Mercilon prevents pregnancy through the same three mechanisms as all combined oral contraceptives. First, the combination of ethinylestradiol and desogestrel suppresses the hypothalamic-pituitary-ovarian axis.

The hormones inhibit the release of follicle-stimulating hormone (FSH) and luteinising hormone (LH) from the anterior pituitary, preventing follicular development and the mid-cycle LH surge that triggers ovulation.

Without ovulation, no egg is available for fertilisation.

Second, desogestrel causes the cervical mucus to become thicker and more viscous, creating a physical barrier to sperm transport.

This effect persists throughout the pill-taking cycle and provides additional contraceptive protection even if ovulation is not fully suppressed on every cycle.

Third, the hormonal environment alters the endometrium, making it thinner and less receptive. This reduces the theoretical likelihood of implantation, though this mechanism is considered a secondary contributor to overall efficacy.

Desogestrel is a pro-drug that is converted in the body to its active metabolite, etonogestrel.

Etonogestrel has high progestogenic potency and low androgenic activity, which is why desogestrel-containing pills may be preferred for women who experience androgenic side effects (acne, hirsutism, oily skin) on levonorgestrel-containing pills.

Clinical evidence and UK prescribing guidance

The contraceptive efficacy of Mercilon has been demonstrated in clinical trials, with a Pearl Index comparable to standard-dose COCs (approximately 0.1 to 0.5 with perfect use).

Studies comparing 20-microgram and 30-microgram ethinylestradiol pills containing desogestrel have found no significant difference in contraceptive efficacy, confirming that the lower oestrogen dose does not compromise pregnancy prevention when pills are taken consistently.

The FSRH guideline on combined hormonal contraception acknowledges that low-dose pills are a valid option and that the choice of pill should be individualised based on the woman's preferences, risk factors, and side effect profile.

The guideline notes that all COCs are highly effective when used correctly and that the VTE risk difference between different formulations, while statistically significant, translates to small differences in absolute risk for most women.

NICE guidance supports shared decision-making in contraceptive choice, with counselling about the comparative risks and benefits of each method.

For women who experience oestrogen-related side effects on standard-dose pills, switching to a 20-microgram pill like Mercilon is a reasonable first step before considering a change in progestogen type or a switch to a non-oestrogen method.

Mercilon compared with other contraceptive pills

Marvelon contains the same progestogen (desogestrel 150 micrograms) with a higher oestrogen dose (ethinylestradiol 30 micrograms).

It may be preferred for women who experience breakthrough bleeding on Mercilon, as the higher oestrogen dose provides better endometrial stabilisation.

Microgynon 30 and Rigevidon (levonorgestrel 150 micrograms with ethinylestradiol 30 micrograms) are second-generation pills with the lowest VTE risk and are recommended by NICE and the FSRH as first-line options when VTE risk is a primary consideration.

Loestrin 20 (norethisterone 1 mg with ethinylestradiol 20 micrograms) is another low-dose pill using a second-generation progestogen.

Femodette (gestodene 75 micrograms with ethinylestradiol 20 micrograms) is a low-dose third-generation pill similar in concept to Mercilon.

Eloine (drospirenone 3 mg with ethinylestradiol 20 micrograms) combines low-dose oestrogen with a progestogen that has anti-androgenic and anti-mineralocorticoid properties.

The progestogen-only pill (POP), such as Cerazette (desogestrel 75 micrograms), is an oestrogen-free alternative for women who cannot or choose not to take oestrogen.

Long-acting reversible contraceptives (LARCs), including the intrauterine system, implant, and injection, offer higher effectiveness with less dependence on daily adherence and are recommended as first-line options by NICE where appropriate.

Dosage and administration

Take one Mercilon tablet at the same time each day for 21 consecutive days. Each tablet in the strip is marked with a day of the week.

After 21 days, stop for 7 days. A withdrawal bleed usually occurs during this interval.

Start the next 21-day strip on day 8, regardless of whether bleeding has finished. Setting a daily alarm or linking pill-taking to a routine activity helps maintain consistency.

Start Mercilon on the first day of your period for immediate protection. If starting at any other time (Quick Start), use condoms for the first 7 days.

Follow FSRH missed pill guidance carefully: one missed pill (less than 48 hours late) requires no additional precautions; two or more missed pills (48 hours or more late) require condoms for 7 days and, if fewer than 7 pills remain, skipping the break and starting a new pack.

Extended regimens (running packs together) are safe and supported by FSRH guidance.

Options include running three packs (63 days) before a 4-day break, or using a 21/4 pattern (shortening the break to 4 days).

These approaches can reduce withdrawal symptoms and may improve long-term cycle control.

Side effects of Mercilon

Common side effects

Headache, breast tenderness, mood changes, nausea, and breakthrough bleeding are the most frequently reported effects.

Breakthrough bleeding is more common in the first 2 to 3 cycles and may be slightly more frequent with 20-microgram pills than with 30-microgram pills.

These effects usually improve with continued use.

Venous and arterial thromboembolism

The risk of VTE with desogestrel-containing COCs is estimated at 9 to 12 per 10,000 women per year.

The risk is highest in the first year and after restarting following a break of 4 weeks or more.

Symptoms of DVT include sudden swelling, pain, and redness in one leg. Symptoms of PE include sudden breathlessness and chest pain.

Call 999 immediately if you experience these symptoms.

Arterial events (heart attack, stroke) are rare but are associated with smoking, hypertension, migraine with aura, and other cardiovascular risk factors.

Other side effects

Changes in libido, bloating, weight fluctuation, vaginal discharge changes, elevated blood pressure, skin changes, and contact lens intolerance may occur.

Rare effects include liver tumours, gallbladder disease, and severe skin reactions. A small increased risk of cervical cancer is associated with long-term use.

The relationship with breast cancer risk is debated but, if present, is very small and disappears within 10 years of stopping.

When to seek urgent medical advice

Stop taking Mercilon and seek urgent medical help if you develop sudden severe headache, visual disturbance, sudden leg pain or swelling, chest pain, breathlessness, or signs of a stroke.

Call 999 in an emergency.

Contact your GP or call NHS 111 for persistent mood changes, new onset migraine, sustained blood pressure elevation, or persistent breakthrough bleeding beyond 3 months.

Report suspected adverse reactions to the MHRA at yellowcard.mhra.gov.uk .

Warnings and precautions

Contraindications

Mercilon must not be used in women with a current or past VTE or arterial thromboembolism, known thrombophilia, uncontrolled hypertension, migraine with aura, diabetes with vascular complications, severe hepatic disease, known or suspected sex-steroid-dependent malignancies, undiagnosed vaginal bleeding, or hypersensitivity to any component.

Risk assessment

Your prescriber should assess blood pressure, BMI, smoking status, VTE and cardiovascular history, and migraine history before starting and annually during use. The UKMEC criteria guide prescribing decisions.

Surgery and immobility

Stop Mercilon 4 to 6 weeks before major surgery with prolonged immobilisation. Restart only after full mobility is restored.

Drug interactions

Enzyme-inducing medicines reduce contraceptive effectiveness. Because Mercilon has a low oestrogen dose, it may be more susceptible to enzyme-inducer interactions than standard-dose pills.

For short-term enzyme-inducer use, additional precautions are needed. For long-term use, an alternative method is recommended. Lamotrigine levels may be altered; discuss with your prescriber if relevant.

Pregnancy and breastfeeding

Do not take Mercilon during pregnancy. If pregnancy occurs, stop immediately. There is no evidence of harm from accidental exposure.

Combined pills are not recommended during breastfeeding in the first 6 weeks postpartum. A progestogen-only method is more suitable while breastfeeding.

How to get Mercilon in the UK

Mercilon is a prescription-only medicine available through the NHS.

You can obtain a prescription from your GP, a sexual health clinic, or an authorised online prescriber registered with the GPhC.

Contraception is free on the NHS throughout the United Kingdom.

If obtained via a standard NHS prescription rather than a contraceptive clinic, the usual prescription charge of 9.90 pounds applies in England; prescriptions are free in Scotland, Wales, and Northern Ireland.

Lifestyle advice while taking Mercilon

No specific dietary restrictions apply. Stop smoking if you smoke, as it substantially increases cardiovascular risk in combination with the pill. Limit alcohol to 14 units per week.

Stay physically active and maintain a healthy weight. Attend cervical screening when invited by the NHS. Be breast-aware and report changes to your GP.

Use condoms alongside Mercilon for protection against sexually transmitted infections.

Sources

• Mercilon Tablets, Summary of Product Characteristics (EMC)
• Desogestrel with ethinylestradiol, British National Formulary (BNF)
• FSRH Clinical Guideline: Combined Hormonal Contraception
• Combined pill, NHS
• MHRA Yellow Card Scheme