I am a little further along (21 months) and I don't recall any unsusual problems with being dizzy or off balance at 18 months. Infections in the inner ear can cause this. Thrush might be a factor. I would run this by your doctor. Depression and irritability are pretty common after effects but once again, 18 months is a ways out. I would speak with a doctor about that also. It's probably nothing ominous, just some things that need to be managed.