Cystelia works as a regular birth control only if it has been taken correctly and consistently, so it may not fully protect you if there are missed or irregular doses. In such cases, an I-pill may be needed as emergency protection after unprotected sex. Taking both together can increase hormone levels and may cause side effects like nausea or irregular bleeding, but it is sometimes used when required. Confirm with a doctor based on how regularly you have been taking Cystelia so you can take the safest step.