Background <p>Diabetes mellitus is a common metabolic disorder. To date, over 589 million adults (1 in 9 people) have been diagnosed with diabetes, and over 3.4 million deaths have occurred annually due to diabetes mellitus. Therefore, early diagnosis and management of diabetes are essential to avoid its microvascular and macrovascular complications. Metformin, an oral biguianide, is considered as a first-line oral antidiabetic agent for patients diagnosed with either prediabetes or diabetes.</p> Case presentation <p>A 56-year-old Iranian male with early-stage diabetes was presented with xerostomia immediately after the first dose of metformin and continued with daily metformin consumption. Metformin-associated xerostomia have not been reported and documented to date. One of the possible causes of metformin-induced xerostomia would be adenosine monophosphate-activated protein kinase overactivation, which may cause reduced saliva production. In addition, mammalian target of rapamycin inhibition may lead to suppression of the salivary gland protein synthesis. Furthermore, metformin-induced Ca²⁺ signaling may result in loss of salivary gland function. In addition, metformin-induced altered insulin sensitivity might be another cause of reduced saliva production.</p> Conclusions <p>Xerostomia and chapped lips are potential adverse drug reactions following metformin administration. Since metformin is a first-line oral antidiabetic agent that is commonly administered to a majority of patients with prediabetes or type 2 diabetes, it is crucial to closely monitor the patients and warn them about these potential adverse reactions for early diagnosis and management.</p>