Does metformin (biguanide oral hypoglycemic agent) cause dryness of mouth (xerostomia)?

Metformin and Dry Mouth

Metformin does not typically cause dry mouth (xerostomia) as a recognized side effect according to current clinical guidelines and evidence.

Side Effect Profile of Metformin

Metformin is the preferred first-line pharmacological agent for type 2 diabetes management, with a well-established safety profile. According to the most recent diabetes care guidelines, the principal side effects of metformin are:

1. Gastrointestinal effects 1:

   • Bloating
   • Abdominal discomfort
   • Diarrhea

2. Other documented side effects 1, 2:

   • Vitamin B12 deficiency with long-term use
   • Potential for lactic acidosis (very rare, primarily in patients with eGFR <30 mL/min/1.73 m²)

Dry Mouth and Medication Relationship

Dry mouth (xerostomia) is a common medication side effect, but metformin is not listed among medications with documented xerogenic effects in comprehensive reviews of medications causing salivary gland dysfunction 3, 4, 5.

The 2017 systematic review sponsored by the World Workshop on Oral Medicine VI compiled an evidence-based list of medications that cause salivary gland dysfunction and xerostomia, and metformin was not included among the 56 chemical substances with higher-level evidence or the 50 substances with moderate-level evidence of causing dry mouth 3.

Managing Gastrointestinal Side Effects of Metformin

While dry mouth is not a typical side effect, the gastrointestinal side effects of metformin can be managed by:

• Starting with a low dose (500 mg once or twice daily) 2
• Gradually titrating to an effective dose (1000-2000 mg daily) 2
• Using extended-release formulations which may reduce GI side effects 1
• Taking metformin with meals 2

Special Considerations

In patients with diabetes experiencing dry mouth, consider:

1. Diabetes itself as a cause: Research has shown that xerostomia is common in diabetic patients (43% in one study), particularly in those with poor glycemic control 6. This appears to be related to the level of HbA1c rather than to medication effects.

2. Other medications: Many patients with diabetes take multiple medications, some of which may cause dry mouth. Review the patient's complete medication list for potential xerogenic drugs 7, 3.

3. Dehydration: Hyperglycemia can lead to polyuria and dehydration, which may contribute to dry mouth symptoms.

Clinical Implications

If a patient on metformin reports dry mouth:

1. Assess glycemic control, as poor control may be the underlying cause
2. Review other medications that may cause xerostomia
3. Evaluate for dehydration
4. Consider other conditions that may cause dry mouth (e.g., Sjögren's syndrome)

Conclusion

Based on the most recent and highest quality evidence, metformin is not recognized as a medication that causes dry mouth. If a patient on metformin experiences dry mouth, other causes should be investigated, including poor glycemic control, other medications, or underlying conditions.