OBJECTIVES: Incidence of unintended pregnancy (UP) is a prominent public health issue and elicits substantial health system costs. The objective of the current analysis was to estimate the direct cost of UP to the National Health Service (NHS) and the proportion of UP costs attributable to poor adherence in the United Kingdom (UK). The cost implications of increased long acting reversible contraception (LARC) uptake were also explored. METHODS: An economic model evaluating costs and outcomes over an averaged one year period of contraceptive usage was constructed. Model inputs were derived from published literature and national survey data collected by the Office of National Statistics. Exploratory analyses were conducted to investigate the impact of women aged 20-29 years switching to LARC methods from existing contraceptive usage. Cost-neutrality analysis was also performed whereby the minimum duration of LARC usage required to achieve a net cost impact of zero post-switch was assessed. RESULTS: Over 400,000 UPs occur annually in the UK at a rate of 34 per 1,000 women. Direct medical costs were estimated to be £382 million annually. Poor adherence accounted for 67% of all UPs, pertaining to costs of £256 million. In women aged 20-29 years, an estimated 213,794 UPs occurred at a cost of £184 million, 64% of which resulted from imperfect adherence. Switching analyses demonstrated substantial cost savings of £12.5 million in women aged 20-29, when 10% switched from oral contraceptives (OCs) to LARC. The duration of LARC usage required to attain cost neutrality for patients switching from OCs was 1.65 years. CONCLUSIONS: The vast proportion of UPs and associated costs in the UK are attributable to poor contraceptive adherence. Increased uptake of LARC methods in young women may generate significant cost savings to UK health care payers in under 2 years.