Metformin is a very commonly used drug used to treat type 2 diabetes mellitus. The latest NHS guidelines (1) point out that this can commonly lead to reduced vitamin B12 levels, which than can lead to many neurological problems, including fatigue, lethargy, pins and needles (paraesthesia), changes in the way that you walk and move around, irritability and depression (2).
If you take metformin, please pay attention to see if you experience these symptoms, and consider having vitamin B12 injections to help. If you suffer from diabetes type 2, make life style changes, specifically a low carb diet, regular exercise and managing stress. In many cases, diabetes type 2 can be reversed and the patient symptom free without medication. This may not be possible in all cases, but it is always worth a try.
(1) https://www.gov.uk/drug-safety-update/metformin-and-reduced-vitamin-b12-levels-new-advice-for-monitoring-patients-at-risk
(2) https://www.nhs.uk/conditions/vitamin-b12-or-folate-deficiency-anaemia/symptoms/