Ethinyl Estradiol
Ethinyl estradiol is the synthetic estrogen used in nearly all combined hormonal contraceptives — pills, patches and vaginal rings. Its high oral potency and long duration make it suitable for once-daily contraceptive dosing combined with a progestin. It is on the WHO Essential Medicines List.
- Chemical formula
- C20H24O2
- CAS number
- 57-63-6
- ATC code
- G03CA01
- Molecular weight
- 296.40 g/mol
- Drug class
- Synthetic estrogen / contraceptive
- Also known as
- EE, EE2, Ethinylestradiol
What is it?
Ethinyl estradiol (EE) is a synthetic estrogen first introduced in the 1940s and the dominant estrogen used in combined hormonal contraceptives — combined oral contraceptive pills (COC), the contraceptive patch (Xulane, Twirla, Evra) and the vaginal ring (NuvaRing, Annovera). It is on the WHO Essential Medicines List and is one of the most prescribed molecules in women's health. EE is paired with various progestins (drospirenone, levonorgestrel, norethindrone, norgestimate, desogestrel) to create the spectrum of combined hormonal contraceptive products.
Mechanism of action
Ethinyl estradiol binds estrogen receptors and produces estrogenic effects similar to natural estradiol. The 17α-ethinyl group blocks first-pass metabolism, giving EE much higher oral bioavailability and longer half-life than estradiol. In contraception, EE works synergistically with the progestin component to suppress LH and FSH release, preventing ovulation. EE also stabilises the endometrium, preventing breakthrough bleeding that would otherwise occur with progestin alone.
Pharmacokinetics
Ethinyl estradiol is well absorbed orally with bioavailability of ~40–45% (limited by enterohepatic recycling and gut metabolism). The terminal half-life is ~24 hours, supporting once-daily oral dosing. Hepatic metabolism is via CYP3A4, which makes EE susceptible to interactions with CYP3A4 inducers (rifampicin, phenytoin, carbamazepine, St John's wort) that can reduce contraceptive efficacy. Patch and ring formulations bypass first-pass metabolism, producing more stable plasma concentrations.
Indications
Ethinyl estradiol is approved as the estrogen component of combined hormonal contraceptives for prevention of pregnancy. It is also used in cyclic combined regimens for menstrual cycle regulation, dysmenorrhoea, premenstrual dysphoric disorder, acne and hirsutism in women, and endometriosis. Bioidentical estradiol is preferred over EE for postmenopausal hormone replacement therapy because of EE's higher thrombotic risk. According to current contraceptive guidelines, the lowest effective EE dose is preferred to minimise venous thromboembolism risk.
Safety profile
Common adverse effects include nausea, breast tenderness, headache, breakthrough bleeding, mood changes and weight changes. The class concern is venous thromboembolism (VTE), which is increased ~3-fold over baseline by combined EE-containing contraceptives, with absolute risk varying by the specific progestin and dose. Smoking, age over 35, hypertension and migraine with aura are contraindications. Lower EE doses (20mcg or below) and modern progestins (levonorgestrel, norethindrone) are associated with lower VTE risk than older 50mcg pills.
Products containing this ingredient
Frequently asked questions
Why is ethinyl estradiol used instead of natural estradiol in birth control? ▾
Ethinyl estradiol has much higher oral bioavailability (~45% vs <5% for natural estradiol) and a longer half-life because the 17α-ethinyl group blocks first-pass hepatic metabolism. This allows reliable once-daily dosing at low doses. In contrast, natural estradiol is used in most modern menopausal HRT because it has a more physiological profile and lower thrombotic risk than EE.
What is the difference between 20mcg, 30mcg and 35mcg EE pills? ▾
Lower-dose pills (20mcg) have lower estrogenic side effects (nausea, breast tenderness) and lower venous thromboembolism risk but more breakthrough bleeding. Higher-dose pills (35mcg) have better cycle control but more estrogenic effects. According to current contraceptive guidelines, starting on 20–30mcg EE is reasonable for most women, with 35mcg reserved for breakthrough-bleeding control.
Can ethinyl estradiol contraception be affected by other medications? ▾
Yes — strong CYP3A4 inducers (rifampicin, certain anticonvulsants, St John's wort, some HIV medications) can reduce EE plasma levels and contraceptive efficacy. According to the prescribing information, women on these medications should use additional or alternative contraception. Most antibiotics do not affect EE contraception, despite older teaching, with rifampicin being a notable exception.
The information on this website is provided for reference and educational purposes only. It does not replace consultation with a qualified healthcare professional.