Personally, I take issue with the grammar of this card. What does the worst of all antibiotics at causing diarrhea even mean. It could mean it is the least likely to cause diarrhea, or it could mean it causes the most diarrhea. The use of a negative for a bad outcome is difficult.
I would prefer “{{c1::Augmentin (amoxicillin and clavulanic acid}} is the least likely to cause diarrhea of all the antibiotics.”
Better yet
The antibiotic: {{c1::Augmentin (amoxicillin and clavulanic acid}} is the least likely to cause diarrhea.