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.