Which method of contraception is considered least effective?

The withdrawal method, commonly known as coitus interruptus, is considered the least effective form of contraception primarily because it relies heavily on the self-control and timing of the male partner to withdraw before ejaculation. This method does not provide any barrier to sperm entering the female reproductive tract and can lead to unintended pregnancies due to pre-ejaculatory fluid, which may contain sperm.

Statistically, the withdrawal method has a relatively high failure rate compared to the other methods listed, with around 22% of typical users experiencing unintended pregnancies over the course of a year. In contrast, the Mirena IUD, condoms, and oral contraceptives have higher effectiveness rates due to their mechanisms, which provide either hormonal control, barrier methods, or both to prevent ovulation and/or fertilization. Thus, while withdrawal may be an option, it is the least reliable and effective method when considering the overall efficacy in preventing pregnancy.