Combined Hormonal Contraception

Overview

Combined hormonal contraception (CHC) — the combined oral contraceptive (COC) pill or the vaginal ring — works primarily by suppressing ovulation, plus thickening cervical mucus. COC and the vaginal ring are equivalent in effectiveness (93% typical use / 99.5% perfect use), contraindications and adverse effects — the ring avoids daily dosing and absorption issues (vomiting/diarrhoea/malabsorption don’t affect it) and causes less breakthrough bleeding, at the cost of device-related symptoms (discharge, expulsion) and no PBS listing.

Contraindications (UK MEC)

Contraindicated (UKMEC 4):

• Current breast cancer
• Breastfeeding, first 6 weeks postpartum (safe after this)
• First 3 weeks postpartum with additional VTE risk factors
• Migraine with aura within the last 5 years (not lifetime — a past episode >5 years ago moves to UKMEC 3, expert judgement)
• Current or past ischaemic heart disease, stroke or TIA
• Aged ≥35 and smoking ≥15 cigarettes/day (or any amount of nicotine vaping, per Australian consensus guidance) — a materially more specific threshold than a blanket “smoker >35yo” rule
• Hypertension with SBP ≥160 or DBP ≥100 mmHg
• Complicated valvular/congenital heart disease
• Current or past VTE; positive antiphospholipid antibodies; known thrombogenic mutation
• Major surgery with prolonged immobilisation
• Severe (decompensated) cirrhosis; hepatocellular adenoma or malignant liver tumour

Requires expert judgement/referral (UKMEC 3):

• BMI >35
• Diabetes with nephropathy/retinopathy/neuropathy/vascular disease
• History of migraine with aura but no episode in the last 5 years
• Multiple cardiovascular risk factors; gallbladder disease
• First-degree relative with VTE <45 years
• 0–3 weeks postpartum without additional risk factors (not breastfeeding); 3–6 weeks postpartum with additional risk factors (not breastfeeding)

Drug interactions

CHC is metabolised via CYP450 — enzyme-inducers reduce contraceptive efficacy:

• St John’s Wort, protease inhibitors, phenytoin, carbamazepine, primidone
• Griseofulvin also reduces CHC efficacy, but by a different, non-CYP450-inducing mechanism (effect persists for at least 28 days after stopping it)
• Antibiotics generally do not affect CHC efficacy except rifampicin/rifabutin

Formulations

Estrogen	Progestogen	Example brands
Estetrol 14200microg	Drospirenone	Nextstellis (newer native-oestrogen option)
Estradiol 1500microg	Nomegestrol	Zoely
Ethinylestradiol 20microg	Levonorgestrel/Drospirenone	Loette, Microgynon 20, Yaz
Ethinylestradiol 30microg	Desogestrel/Dienogest/Drospirenone/Gestodene/Levonorgestrel	Marvelon, Valette, Yasmin, Minulet, Microgynon 30
Ethinylestradiol 35microg	Cyproterone/Norethisterone	Diane-35, Norimin
Ethinylestradiol 50microg	Levonorgestrel	Microgynon 50 — not recommended for contraception (unacceptable VTE risk); role limited to short-term acute heavy uterine bleeding
Vaginal ring: Ethinylestradiol + Etonogestrel	—	NuvaRing

First-line choice: a monophasic pill with ethinylestradiol 20–30microg + levonorgestrel — inexpensive, PBS-listed, similar discontinuation to other options. 20microg causes more breakthrough bleeding than 30/35microg but has a slightly lower VTE/cardiovascular risk. Estradiol/estradiol valerate/estetrol (“native” oestrogens) have less effect on VTE laboratory markers than ethinylestradiol, but cohort data have not yet shown a clear reduction in actual VTE/cardiovascular events.

Cyproterone-containing pills are TGA-approved only for hirsutism/severe acne (not first-line contraception) and cannot be used for Quick Start unless pregnancy is reasonably excluded (theoretical risk of male fetal feminisation).

Starting CHC

Immediately effective if:

• Started day 1–5 of a regular cycle
• <21 days postpartum, or ≤5 days post-abortion
• Switching COC-to-COC/ring (new pack/ring started no more than 24hrs after the end of the hormone-free interval)
• Switching from Implanon or Depot (on/before removal or when injection due)
• Switching from copper IUD, days 1–5 of cycle

Otherwise, CHC takes 7 days to become effective — use barrier contraception for that week.

Quick start: starting on the day of consult regardless of cycle timing — useful for long/irregular cycles, when unintended pregnancy would be dangerous, or poor healthcare access. Early pregnancy cannot be excluded — pregnancy test on the day, repeated at 4 weeks (or 3 weeks if no unprotected sex in the first week).

Missed pills / deviations

• COC is not “missed” until >24hrs late (48hr maximum interval between doses).
• <24hrs late: take immediately; continues to work.
• >24hrs late: take the most recently missed pill, discard others, continue active pills as usual (skip the inactive/placebo week if within 7 days of it); use condoms for 7 days; consider levonorgestrel emergency contraception if <7 active pills taken since the last hormone-free break and unprotected sex in the last 5 days.
• Applies also to drospirenone-only POPs (e.g. Slinda). Does not apply to Qlaira (quadriphasic — different, more complex rules) or if ≥5 pills missed (seek advice).
• Vaginal ring: late reinsertion/early removal follows an analogous algorithm — >24hrs late requires condoms for 7 days ± emergency contraception depending on ring-use duration; a ring left in place >4 weeks needs a fresh ring plus 7 days condoms.

Tailored regimens (extended/continuous use)

Monthly withdrawal bleeds have no clinical benefit — tailoring is safe and effective, and is specifically useful for dysmenorrhoea, endometriosis, heavy menstrual bleeding, or POI symptoms during the hormone-free interval.

• Continuous use: no hormone-free interval at all
• Extended use/tricycling: 9–12 weeks of active pill/ring, then a 4–7 day break
• Shortened hormone-free interval: e.g. 24 active days + 4 inactive (Yaz, Zoely)

Any monophasic CHC licensed for standard dosing can be used this way; there’s no upper limit on how many hormone-free intervals can be skipped. If breakthrough bleeding is troublesome (≥4 days) on continuous use, a 4-day break is reasonable once ≥21 days of active use have passed. Persistent unexplained bleeding still needs the usual work-up (STI, pregnancy, enzyme-inducing drugs, structural pathology).

Managing adverse effects

Symptom	Action
Headache	Reduce oestrogen/progestogen dose; if in the hormone-free interval, try a tailored regimen
Nausea	Reduce oestrogen; take at night; consider switching method
Breast tenderness	Reduce dose; consider a drospirenone-containing pill
Bloating/fluid retention	Reduce dose; drospirenone has a mild diuretic effect
Breakthrough bleeding	Exclude other causes first; if on 20microg ethinylestradiol, consider a higher dose; change progestogen or switch to the ring
Mood changes	Medical review if new/worsening; address other causes; consider tailored regimen or drospirenone if premenstrual
Acne/reduced libido	Evidence to guide formulation choice is limited

Most symptoms settle within 3 months — reasonable to wait before switching, once other causes are excluded. Persistent nausea should prompt a pregnancy test. New/first migraine on CHC warrants expert review of ongoing suitability.