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The use of antibiotics usually affects the efficacy of the oral contraceptives, patches or Nuvaring. These can affect efficacy of injectable forms also (although rarely seen). If you had been sexually active (without additional protection) during your antibiotic treatment, then chances of pregnancy could be possible. Since none of the birth control measures give you 100% protection, you should use back-up protection while having intercourse for additional protection from pregnancy (if you are worried about pregnancy). Visit your doctor/gynecologist for further discussion and evaluation.
Take care.