Dry mouth, medically referred to as xerostomia, is the most common side effect of phentermine. Weight loss medications like phentermine promote this feeling of dryness in the mouth, lips, and tongue due to their stimulant effect. Severity varies from person to person, but most patients report improvement in symptoms after about 1-2 weeks. Staying hydrated can also help alleviate some of the most bothersome symptoms.
→ To learn about other possible reactions to phentermine, read this extensive review of phentermine side effects.