Authors: Sakamoto LC, Melo NR, Pompei LM, Tobara JC, Malvasi A, Gebrin LH. Objective: to evaluate harm reduction with the prevention of unplanned pregnancy through contraceptive implantation in women using illicit drugs in the region of Cracolândia, São Paulo. Methods: 235 etonogestrel subdermal implants (Implanon®) were inserted by the Women's Health Reference Center, voluntarily, in women users of illicit drugs in the central region of the city of São Paulo, between March 2014 and June 2017. Four important variables were observed for the prevention of unplanned pregnancy in this population: Why is it a vulnerable population? Why do you get pregnant? When you get pregnant, why is it a high-risk pregnancy? And, what damages are reduced with the prevention of unplanned pregnancy? Results: The mean age of the women who underwent implantation was 25.7 years, represented by 80 black women (34%), 80 brown women (34%) and 75 white women (32%). These women are vulnerable, because 74 are in a street situation (31%), 85 prefer to use crack as a preferential drug (36%), 181 are associated with tobacco (77%) and 147 with alcohol (63%), and 78 of them exchange sex for drugs or money to buy drugs (33%). They are easy to conceive because 65 of them do not use any contraceptive method (28%), 81 (48%) prefer to use the condom as a method, being the most used, however, often do not do it correctly. Regarding the obstetric history, we have 18% of abortions, 1.3% of intrauterine fetal death and 18.5% of preterm deliveries. Finally, drugs cause social and mainly family damages, where 77% of these children do not live with their mothers and many of them are in shelters or in an unknown situation. The care with this population falls on the public service, which saves R$ 340,000.00 (three hundred and forty thousand reais) per year for every 100 women with the prevention of these pregnancies. No pregnancy has been observed to date. Conclusion: etonogestrel subdermal contraceptive implant is the most appropriate method in women who use illicit drugs due to the reduction of harm to the prevention of unplanned pregnancies.