Are there any serious downsides to taking expired pepto bismol (bismuth subsalicylate)? the expiration date says october 2015 and it is november 9th

No: It is probably just as good today as it was last month. Expiration dates are generally somewhat arbitrary and most medicines are still safe and effective for several months after the expiration date. Sometimes they may lose some potency over time but, from my understanding of the main ingredient in Pepto Bismol, bismuth subsalicylate, it should be perfectly fine to take it today.