Can phentermine (phentermine) cause constipation?

Does Phentermine Cause Constipation?

Yes, constipation is a common side effect of phentermine, occurring in approximately 15-17% of patients taking the medication for weight loss. 1

Evidence on Phentermine and Constipation

According to the American Heart Association's guidelines, constipation is listed among the most common side effects of phentermine, along with dry mouth and insomnia 1. This finding is consistent across multiple high-quality sources:

• The 2017 Gastroenterology guidelines specifically mention constipation as one of the most common side effects of phentermine/topiramate ER combination therapy 1
• In the CONQUER trial, constipation was reported in 15% of patients taking phentermine 7.5 mg plus topiramate 46.0 mg and 17% of patients taking phentermine 15.0 mg plus topiramate 92.0 mg, compared to only 6% in the placebo group 2

Mechanism and Management

Phentermine is a β-phenethylamine derivative that stimulates the release of norepinephrine and dopamine from nerve terminals 1. This sympathomimetic activity not only suppresses appetite but can also affect gastrointestinal motility, potentially leading to constipation.

Managing Phentermine-Associated Constipation:

1. Adequate hydration: Ensure sufficient water intake throughout the day
2. Dietary fiber: Increase consumption of high-fiber foods
3. Regular physical activity: Maintain consistent exercise to promote bowel motility
4. Consider fiber supplements: May help alleviate constipation while on phentermine

Clinical Considerations

Risk Factors for Constipation with Phentermine:

• Pre-existing tendency toward constipation
• Inadequate fluid intake
• Low-fiber diet
• Sedentary lifestyle

When to Be Concerned:

• Severe or persistent constipation
• Abdominal pain with constipation
• Blood in stool
• New onset of constipation with abdominal pain

Important Caveats

While constipation is a common side effect, it's worth noting that:

1. The constipation rate is higher with combination therapy (phentermine/topiramate) than with phentermine alone
2. Constipation is generally manageable and rarely leads to discontinuation of therapy
3. In rare cases, severe constipation could potentially progress to more serious gastrointestinal issues 3

Monitoring Recommendations

For patients starting phentermine:

• Discuss the possibility of constipation before initiating therapy
• Recommend preventive measures (adequate hydration, fiber intake)
• Follow up within 2-4 weeks to assess for side effects including constipation
• Consider dose adjustment if constipation is severe or persistent

Phentermine is typically prescribed for short-term use (12 weeks), which may limit the duration of constipation symptoms for most patients 1.