Several things. Nausea, vomiting, epigastric pain, or more rarely diarrhea. Tinnitus, headache and gastrointestinal bleeding are also possible. In more serious poisoning, toxicity is seen in the central nervous system, manifesting as drowsiness, occasionally excitation and disorientation or coma. Occasionally patients develop convulsions. http://www.mhra.gov.uk/howweregulate/medicines/licensingofmedicines/inform.
"Poison" Dr. Isidor rosenfeld's grandmother used to say in every medicine there's a little poison. For maximum effectiveness with minimal side effects all medications should be taken exactly as directed.