For many years it was widely advised that all antibiotics could make the contraceptive pill fail, but current evidence shows that the vast majority of antibiotics, including amoxicillin, do not affect how well the pill works. The main exceptions are enzyme-inducing antibiotics such as rifampicin and rifabutin, which do reduce its effectiveness.
The more practical risk is that being unwell with vomiting or diarrhoea can stop the pill from being absorbed properly, which can reduce protection regardless of the antibiotic. In that case, follow the missed-pill or extra-precaution advice in the patient leaflet.
If you are prescribed rifampicin, are vomiting, or are unsure, use additional contraception such as condoms and ask your pharmacist or doctor for tailored advice.