When infants get vaccinations, they often get a fever from it, so doctors often give them acetaminophen (eg Tylenol) as a prophylaxis, but it turns out the acetaminophen can reduce the effectiveness of the vaccine. In this controlled study in the Czech Republic, published in the Lancet, 10% of the kids who got acetaminophen to control the fever "had significantly lower levels of protective antibodies against the targets of the vaccination." Fewer kids who got acetaminophen did get a fever (42% vs 66%), so it worked against fever for an extra 24% of the kids. LA Times summary here.
This suggests parents and doctors should wait until a fever develops before giving an infant acetaminophen.