This prospective study aimed to analyze whether the patients with pre-diabetes (pre-DM) reach the TC (therapeutic concentration) of the metformin during repeated, low, constant drug dose. The guidelines do not recommend any metformin dose for this group of patients. Based on the previous study after a dose of 1700 mg/day the patients seem to reach the therapeutic drug concentration, which guarantees the glycemic effect. Twenty patients with new-diagnosed pre-DM were treated with a 1500 mg/day regimen of the metformin for 15 weeks. The serum concentration of the drug was assessed by liquid chromatography-mass spectrometry technique at 6 and 15 week of the treatment. The correlation of the serum metformin concentration with BMI (body mass index) and patients' weight was also performed. The mean metformin concentration was: 4.65 μmol/L (± 2.41) and 5.41 μmol/L (± 3.44) (p = 0.27) after 6 and 15 weeks of the treatment respectively. There was a positive correlation between the serum concentration of the metformin and body weight (but not BMI) in the 15th week of the therapy (p = 0.04)- the higher body weight the higher concentration of the metformin. Patients with pre-diabetes can be successfully treated with a low dose of metformin, to reach the drug's therapeutic concentration. Body weight can impact the metformin serum concentration during long-term treatment what should be taken into consideration when choosing the dose because of its pleiotropic effect e.g. on the cardiovascular system via reduction of the oxidative stress and would be not connected with the drug's hypoglycemic effect.ClinicalTrials.gov number: NCT03398356; date of first registration: 01/07/2018.