I had 5 days on antibiotics last week - I am SSing and found that I didnt lose anything until I had finished the course - however I did also have to take ibuprofen which I know can cause water retention. The tablets didnt knock me out of ketosis, and I am very carb-sensitive. Once I'd stopped taking everything the scales started moving properly again. I'd say dont worry about taking them, but ensure you drink up to 4l water as I found this really helped to get things moving again.