Common side effects of generic Principen to be aware of are nausea, vomiting, loss of appetite, diarrhea, rashes, itching, and headache. Patients with a history of allergic reactions to other penicillins should not buy Principen. More serious but rarely reported conditions with this medication include seizures, anaphylaxis, and a lowering in the patient's red blood cell count. Contact a local physician immediately if there are signs of swelling in the mouth or face, trouble breathing, tightness in the chest, bloody stools, or stomach cramps as these symptoms could be signs of allergic reactions.