Yes it is true that penicillin group of drugs can cause decrease in contraceptive efficacy of hormonal implants as well. If you have been on high dose of amoxyxillin for your infection then there are chances that you might experience a breakthrough bleed in this cycle and your protection is jeopardized.
It is advisable that you may use barrier contraception as additional form of contraception during next 3 weeks as it would take around 2 weeks to re-establish control after stoppage of antibiotics.
I hope i have answered your query in detail,
wishing you good health,
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