When is a rash in reaction to augmentin (amoxicillin and clavulanate) more severe than a "side effect?" i have a rash that is very itchy and spreading.

Stop drug, call doc: There is no difference between "side effect" and a severe reaction. Any side effect can be mild or severe. An itchy rash may not seem dangerous, but sometimes can progress and become serious very quickly. Stop the Augmentin (amoxicillin and clavulanate) immediately and contact the doctor who prescribed it. Most likely s/he will switch to a different antibiotic, and you may need to avoid penicillin-related drugs in the future.