hiya yorkiegirl, I have not taken long-term antibiotics personally but my dd was on them for a year (from age 2 - 3) for respiratory infections, mostly full dose but occasionally low dose. It really made a lot of difference to her, of course she would still catch the odd cold but it wouldn't develop into loads of secondary infections and stay quite mild. She never displayed any side effects.
How is your asthma? Might it be worth upping your preventer usage come autumn?