<prompt> All of the statements regarding progesterone only pill are true, exceptInhibits ovulationFailure rate is the same as combined OCPIrregular bleeding is a known complicationActs by altering the cervical mucous  <response>: Failure rate of combined OCP is 0.2100 women years, whereas failure rate of progesterone only pill is about 0.5 - 2 per 100 women years of use. Progesterone only pill acts by thickening cervical mucous, causing atrophy of endometrium or by inhibiting ovulation. 