Wizzette,
what would concern me more than the actual one dose you took is how deliberate you were in not letting the two people with you see that you got that syrup. how deliberately you only got the antibiotic when they were with you and went back later, alone, to get the syrup with the codeine. you were hiding it.
and now you wish to hide some more...
for me, honesty with self grew slowly. and when i was honest with myself about certain things, phew, that was good and good enough. the understanding of the value of honesty with someone else, the value of overcoming fear and speaking of how things are, the value of the relief of not hiding...took a loooong time.
i'd guess that you wouldn't have posted about it if you felt okay about it all. your gut (or inner voice or conscience or better self or plain good sense or whatever you want to call it) is telling you things are off with this scenario.