Generally, phentermine appears to be relatively well tolerated It can produce side effects consistent with its catecholamine-releasing properties, e.g., tachycardia (increased heart rate) and elevated blood pressure, but the incidence and magnitude of these appear to be less than with the amphetamines. Because phentermine acts through sympathomimetic pathways, the drug may increase blood pressure and heart rate. It may also cause palpitations, restlessness, and insomnia. Its less common, but more severe, side effects include:[citation needed] Convulsions (seizures) Fever Hallucinations Hostility with urge to attack Bizarre behavior Mental or mood changes Exaggerated sense of well-being Irregular blood pressure Severe or persistent light-headedness, fainting or headache Periods of mania followed by period of depression Fast or irregular heartbeat Overactive reflexes Tremors, trembling or shaking Panic Restlessness Severe nausea, vomiting or diarrhea Stomach cramps Weakness Constipation Primary pulmonary hypertension Regurgitant cardiac valvular disease Pounding in the chest or shortness of breath. Your symptoms can be because of high dose of the drug because of the sympathomimetic activity and may be the dose needs to be reduced. Thanks.