My now 4-y-o son has swelled up terribly at the site of mosquito bites on his head. He was given antibiotics on two occasions when the swelling involved his eye and the ped. was afraid of an infection there. As long as the reaction remains "local" and there's no trouble breathing, for instance, it doesn't seem to be so serious, just uncomfortable. My son was pretty groggy for days (starting even before I had to get the antibiotic in him).