Can someone overdose on too much phentermine?

Yes. You can overdose on phentermine, and it can be fatal. Overdose symptoms vary, but may include feeling hostile or aggressive, nausea, vomiting, diarrhea, stomach cramps, irregular heartbeat, rapid breathing, overactive reflexes, confusion, hallucinations, seizure (convulsions), feeling light-headed, or fainting. If you experience these symptoms after taking Phentermine please seek immediate help.