Erythromycin has a short half life in dogs, so it's usually given IV or orally three times a day. It has a little higher incidence of side effects (not so serious ones) so it's not used often any more as there are other choices better suited. Back in my early years of practice, it was a pretty effective antibiotic to use in some situations. Not a placebo at all.
The lettuce being toxic is a internet perpetuated false statement (FAKE NEWS :B)
The only peanut butters that cause a problem are ones with xylitol added as BizmarksMom stated.
Doug, DVM