The following information is directly taken from the patient leaflet of Nexplanon:
Switching hormonal contraceptive method to Nexplanon
Changing from a combined hormonal contraceptive method (combined oral contraceptive (COC), vaginal ring or transdermal patch).
The implant should be inserted preferably on the day after the last active tablet (the last tablet containing the active substances) of the previous combined oral contraceptive or on the day of removal of the vaginal ring or transdermal patch. At the latest, the implant should be inserted on the day following the usual tablet-free, ring free, patch free or placebo tablet interval of the previous combined hormonal contraceptive when the next application would have been due. Not all contraceptive methods (transdermal patch, vaginal ring) may be available in all countries.
If inserted as recommended, back-up contraception is not necessary. If deviating from the recommended timing of insertion, the woman should be advised to use a barrier method until 7 days after insertion. If intercourse has already occurred, pregnancy should be excluded.
Changing from a progestagen-only contraceptive method (e.g. progestagen-only pill, injectable, implant, or intrauterine system [IUS])
As there are several types of progestagen-only methods, the insertion of the implant must be performed as follows:
• Injectable contraceptives: Insert the implant on the day the next injection is due.
• Progestagen-only pill: A woman may switch from the progestagen-only pill to Nexplanon on any day of the month. The implant should be inserted within 24 hours after taking the last tablet.
• Implant/Intrauterine system (IUS): Insert the implant on the same day the previous
implant or IUS is removed.
If inserted as recommended, back up contraception is not necessary. If deviating from the recommended timing of insertion, the woman should be advised to use a barrier method until 7 days after insertion. If intercourse has already occurred, pregnancy should be excluded.
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I am fairly certain Slinda is a mini pill so you wouldn’t have been covered right away. It takes 7 days for your implant to take effect. EllaOne also cancels out birth control for a while after taking it. The situation sounds a bit messy so it might be best to take a pregnancy test 21 days after the sex for accurate results.