Combined Hormonal Contraception: Mechanism, Risks and Who Can't Take It

What is in the pill, the patch and the ring

"Combined" means two hormones together: an oestrogen and a progestogen. Combined hormonal contraception delivers a synthetic oestrogen — almost always ethinylestradiol — together with a progestogen (a synthetic cousin of progesterone). The same two-hormone recipe comes in three delivery forms: the combined oral contraceptive pill (a daily tablet), the transdermal patch (changed weekly), and the vaginal ring (worn for three weeks). The route differs but the pharmacology is the same, and — this is the point students miss — so do the risks. The patch and ring are not "safer" alternatives that dodge the oestrogen problem; they carry the same oestrogen and the same cautions. These are the very molecules taught in the Endocrine section: here we are not re-learning what oestrogen and progesterone are, but applying them as drugs.

The main mechanism: silencing the HPG axis

Normally the hypothalamus releases GnRH in pulses, which drives the pituitary to secrete FSH and LH; FSH grows an ovarian follicle, and a mid-cycle surge of LH triggers ovulation. Combined contraception hijacks the body's own negative feedback. Steady daily oestrogen and progestogen tell the hypothalamus and pituitary that hormone levels are already "high", so GnRH pulsing is suppressed and FSH and LH stay low and flat. Without an FSH rise, no dominant follicle matures; and — the decisive step — without the mid-cycle LH surge, there is no ovulation. No egg is released, so there is nothing to fertilise. This is the same hypothalamic–pituitary–gonadal (HPG) axis and negative-feedback logic taught in the Endocrine chapters, turned into a therapeutic tool.

Suppressing ovulation is the headline, but two backup mechanisms add belt-and-braces protection. The progestogen thickens cervical mucus into a barrier that sperm struggle to penetrate, and it thins the endometrium so that even in the rare event of ovulation and fertilisation, the lining is inhospitable to implantation. So the pill works at three levels — no egg, a hostile gateway, and an unreceptive lining — which is why, taken correctly, it is extremely effective. Its real-world failure rate is driven almost entirely by missed or late pills, not by the drug failing to do its job.

The benefits beyond contraception

Because it imposes a regular, controlled hormonal rhythm, combined contraception does far more than prevent pregnancy. It gives predictable, lighter and less painful withdrawal bleeds — so it is a genuine treatment for heavy menstrual bleeding and dysmenorrhoea, a link to the Heavy menstrual bleeding chapter. By suppressing ovarian androgen production and raising sex-hormone-binding globulin, it improves acne and hirsutism, which is why it is often chosen in polycystic ovary syndrome. Over years of use it meaningfully lowers the lifetime risk of ovarian and endometrial cancer — a protective effect that persists after stopping. These non-contraceptive benefits are real prescribing reasons in their own right, not just happy side effects.

The dominant issue: oestrogen and who cannot take it

Almost every serious risk of the combined pill traces back to one molecule: the oestrogen. Oestrogen is prothrombotic. It nudges the liver to make more clotting factors and shifts the balance of the coagulation system towards clot formation, which is why the headline risk of combined contraception is venous thromboembolism (VTE) — deep vein thrombosis and pulmonary embolism. The absolute risk in a healthy young woman is still small (pregnancy itself carries a higher VTE risk than the pill), but it is real, and it multiplies when other risk factors stack on top: obesity, smoking, prolonged immobility, a known thrombophilia, a family history of VTE, and the early postpartum period. This is the same clotting cascade and prothrombotic logic developed in the Haematology section — here the drug is tipping that balance. Separately, the oestrogen drives arterial risk too — ischaemic stroke and myocardial infarction — and it can raise blood pressure, so BP must be measured before starting and monitored on treatment.

This is where the UK Medical Eligibility Criteria (UKMEC) framework earns its place. Rather than a vague sense of "risk", UKMEC grades each condition from 1 (no restriction) to 4 (an absolute contraindication — do not use). Two categories 4 dominate the exam. The first is migraine with aura: the aura reflects a transient neurological disturbance, and adding oestrogen's prothrombotic effect meaningfully raises the risk of ischaemic stroke — so migraine with aura is an absolute contraindication at any age. The second is the smoker aged 35 or over, especially at 15 or more cigarettes a day, where the combined arterial risk of age, smoking and oestrogen becomes unacceptable. Other UKMEC 4 or 3 situations include a personal history of VTE or arterial disease, a known thrombophilia, uncontrolled hypertension, current breast cancer, severe (decompensated) liver disease, and the first six weeks postpartum in a breastfeeding mother (both a VTE concern and an effect on milk).

Progestogen generation and the VTE nuance

The oestrogen carries most of the VTE risk, but the choice of progestogen fine-tunes it. Older "second-generation" progestogens such as levonorgestrel carry the lowest VTE risk, while some newer progestogens (for example desogestrel, gestodene and drospirenone) are associated with a modestly higher VTE risk. The difference is small in absolute terms, but it is why guidance often favours a levonorgestrel-containing pill as a sensible first choice, reserving the newer progestogens for when their particular properties — for instance the anti-androgenic effect of some, useful in acne — are actually wanted. The pragmatic rule: use the lowest effective oestrogen dose with a lower-VTE-risk progestogen unless there is a specific reason to do otherwise.

Drug interactions: the enzyme inducers

The combined pill is metabolised in the liver, so anything that revs up hepatic enzymes can quietly render it useless. Hepatic enzyme inducers — drugs that upregulate cytochrome P450 — speed the breakdown of ethinylestradiol and the progestogen, dropping hormone levels below the contraceptive threshold and risking pregnancy. The classic offenders are rifampicin (and rifabutin), several older antiepileptics — carbamazepine, phenytoin, and also topiramate — and the herbal remedy St John's wort. This overlaps directly with the Central Nervous System section, where those enzyme-inducing antiepileptics are taught: a woman on carbamazepine cannot rely on a standard combined pill. The practical response is to use a method that enzyme inducers don't undermine — most reliably the intrauterine device or the injectable — or, for short courses, add extra precautions. Note the historical correction that trips up students: ordinary (non-enzyme-inducing) antibiotics such as amoxicillin are no longer considered to reduce combined-pill efficacy; only the enzyme-inducing antibiotics like rifampicin genuinely matter.

Missed pills, side effects, and the 7-day principle

Because the pill's grip on the HPG axis relaxes if the hormone supply is interrupted, the missed-pill rules revolve around a single idea: it takes about seven consecutive days of consistent pills to reliably suppress ovulation, and roughly the same gap without hormone to let a follicle escape. So one missed pill is easily rescued — take it as soon as remembered and carry on — but two or more missed pills, especially near the hormone-free interval, threaten ovulation and call for extra precautions (barrier method) for seven days, with emergency contraception considered if there has been unprotected sex. The commonest early side effects are breakthrough (unscheduled) bleeding — usually settling within the first few months — along with breast tenderness, nausea, headaches and mood changes. Most are mild and transient; the clinician's job is to distinguish these nuisance effects from the rare warning signs of a serious oestrogen complication — the calf pain of a DVT, the breathlessness of a PE, a sudden severe headache or focal neurological symptoms.