All drugs have potential side effects, and nausea and vomiting are common ones. This would not be an allergic reaction but a side effect. Allergies result in rashes, hives, wheezing and serious reactions shortly after taking the medicine. She might be having the vomiting related to her flu-like illness, not necessarily the medicine. Penicillin is not normally given for flu, so she should check with her doctor to be sure she really needs it, and if so, perhaps try another antibiotic that won t cause her side effects. Hope this helps.