BACKGROUND Unprotected intercourse after oral emergency contraception (EC) significantly increases pregnancy risk. This underlies the importance of promptly starting effective, ongoing contraception - known as 'quick starting'. However, theoretical concern exists that quick starting might interact with EC or hormonal contraception (HC) potentially causing adverse side effects. METHODS A systematic review was conducted, evaluating quick starting HC after oral EC [levonorgestrel 1.5 mg (LNG) or ulipristal acetate 30 mg (UPA)]. PubMed, EMBASE, The Cochrane Library, ICTRP, ClinicalTrials.gov and relevant reference lists were searched in February 2016. A lack of comparable studies prevented meta-analysis. RESULTS Three randomised controlled trials were identified. Two biomedical studies suggested HC action was unaffected by quick starting after UPA; one study examined ovarian quiescence (OR 1.27; 95% CI 0.51-3.18) while taking combined oral contraception (COC). Another assessed cervical mucus impenetrability (OR 0.76; 95% CI 0.27-2.13) while taking progestogen-only pills (POP). Quick starting POP reduced the ability of UPA to delay ovulation (OR 0.04; 95% CI 0.01-0.37). Side effects (OR 1.22; 95% CI 0.48-3.12) and unscheduled bleeding (OR 0.53; 95% CI 0.16-1.81) were unaffected by quick starting COC after UPA. Another study reported higher self-reported contraceptive use at 8 weeks among women quick starting POP after LNG, compared with women given LNG alone (OR 6.73; 95% CI 2.14-21.20).