Is there no research evidence that common antibiotics lower birth control pill effectiveness? drug-related diarrhea would lower effectiveness, right?

Plenty of research: There is a great deal of research on the effects of antibiotics on oral contraceptive reliability. Not all of it is consistent, though. It is clear that rifampin and griseofulvin are two antibiotics that definitely decrease the reliability of OC's. With other antibiotics, the evidence is not so clear. It is prudent to discuss this with your doctor when you're prescribed antibiotics. Good luck.

No: No evidence that is true. even diarrhea should not change that.

Antibiotics and BCP: yes, tetracyclines and ampicillins may decrease hormonal contraceptive efficacy, so better use alternate contraceptive method while antibiotics are prescribed, not sure if diarrhoea does as well decrease efficacy,