The half-life of UPA is hours, which means that most of the drug is eliminated by 1 week and the interaction with hormonal contraception is likely to be clinically insignificant by this time. If a hormonal method is quick started the CEU advises use of additional contraceptive precautions for 1 week after taking UPA plus the time required for contraceptive efficacy to be established (see Summary on page iv). There is also a theoretical concern that progestogen-containing contraceptives could antagonise the action of UPA if taken concurrently or started soon after administration of UPA. Although there is no evidence looking at this interaction, UPA has been.
