Phentermine Side Effects
Phentermine causes common side effects including dry mouth, constipation, insomnia, palpitations, and irritability, with rare but serious cardiovascular risks including primary pulmonary hypertension, valvular heart disease, and elevated blood pressure and heart rate. 1
Cardiovascular Side Effects
The most concerning adverse effects involve the cardiovascular system:
- Primary pulmonary hypertension (rare but serious) - requires immediate discontinuation if new dyspnea, angina, syncope, or lower extremity edema develops 1
- Valvular heart disease - serious regurgitant cardiac valvular disease affecting mitral, aortic, and/or tricuspid valves has been reported, though the role of phentermine alone (versus combination with fenfluramine) remains unclear 1
- Palpitations and tachycardia - commonly reported; heart rate increases by 5-10 beats per minute 2, 1, 3
- Elevated blood pressure - though observational data suggest blood pressure generally does not increase significantly with monotherapy 2
- Ischemic events - rare cases reported 1
- Atrial fibrillation - rarely reported but documented 4
Blood pressure and heart rate should be monitored periodically, especially in the first 12 weeks of treatment 2. The medication should be avoided in patients with cardiovascular disease or uncontrolled hypertension 2.
Central Nervous System Side Effects
Neuropsychiatric effects are common, particularly at higher doses:
- Insomnia - very common; dose should be taken early in the day to minimize this risk 2, 5
- Overstimulation and restlessness 1
- Dizziness - reported in 10% at higher doses 5, 6
- Headache 1
- Tremor 1
- Euphoria or dysphoria 1
- Psychosis - rare 1
- Depression and anxiety - when combined with topiramate, depression-related adverse events occurred in 4-7% and anxiety-related events in 3-8% 5
- Disturbance in attention and irritability - particularly with phentermine-topiramate combination 6
Gastrointestinal Side Effects
GI effects are among the most frequently reported:
- Dry mouth - occurs in 13-21% depending on dose 5, 6
- Constipation - reported in 15-17% 2, 5, 6
- Unpleasant taste (dysgeusia) - occurs in 7-10% 5, 6
- Diarrhea 1
- Other GI disturbances 1
- Ischemic colitis - extremely rare but documented 7
Other Side Effects
- Paresthesia - particularly with topiramate combination (14-21%) 5, 6
- Hypoesthesia - numbness or reduced sensation 6
- Urticaria (allergic reaction) 1
- Impotence and changes in libido 1
- Fatigue 3
Important Clinical Considerations
The medication impairs the ability to operate machinery or drive 1. Phentermine is a Schedule IV controlled substance due to its chemical similarity to amphetamines, though recent data suggest addiction potential is low 8.
Concomitant alcohol use may result in adverse drug reactions 1. The medication is contraindicated with monoamine oxidase inhibitors (within 14 days) due to risk of hypertensive crisis 1.
In patients with diabetes, insulin or oral hypoglycemic medication doses may need reduction 1. The medication should be avoided in patients with untreated hyperthyroidism due to concerns for arrhythmias and seizures 2.
Dose-Related Effects
Side effects are dose-dependent. The phentermine-topiramate combination shows increasing adverse event rates with higher doses: dry mouth (2% placebo vs 13% at 7.5/46mg vs 21% at 15/92mg), paresthesia (2% vs 14% vs 21%), and constipation (6% vs 15% vs 17%) 5. Gradual dose titration helps mitigate adverse effects 2.