Spotting is bleeding between 2 menstrual cycles due to hormonal imbalance in the body due to the use of hormonal birth control that is used by the community, namely 3-month injection birth control. Spotting occurs due to an increase in the amount of the hormone progesterone in the body due to injections. This study aimed to determine the effect of the duration of using 3 months of injectable birth control on the incidence of spotting. The research method uses an analytical survey with a retrospective approach. A sample of 246 KB injection participants for 3 months was taken by systematic simple random sampling. Statistical analysis using Fisher's Exact test. The results showed that most of the 3-month injection KB participants who were less than 2 years old experienced spotting side effects, while those who used injection KB for more than 2 years did not experience spotting. A p-value of 0.000 was obtained (p-value <0.05) so that there was an effect of the duration of using 3 months of injection contraception on the incidence of spotting. It is hoped that this research can be used for PUS who will use 3-month injectable birth control or who are currently using 3-month injection birth control as an illustration of the side effects that will be experienced when using 3-month injection birth control, so there are no dropouts and switching to using non-hormonal birth control.