Male infertility has been increasing over the last decades being nowadays a 
pressing health problem. Diabetes mellitus (DM) can contribute directly or 
indirectly to male infertility due to an abnormal spermatogenesis, which results 
in a decreased sperm quality. Type 2 Diabetes mellitus (T2DM) is responsible for 
the vast majority of DM cases, being frequently treated with oral antidiabetic 
drugs. Metformin is the most cost-effective therapy for the treatment of T2DM. 
This biguanide is an oral insulin-sensitizing agent capable of increasing 
insulin sensitivity and decreasing plasma fasting insulin levels. The main 
metabolic action of this drug occurs in the liver. However, it has been shown 
that metformin acts on a variety of organs including the male reproductive 
system. With the rising numbers of diabetic individuals among younger 
populations, there is an increase in the consumption of metformin in individuals 
of this age group. As a result, it is important to discuss the role of metformin 
in male fertility. This review presents the most recent data available from 
studies on the effects of metformin on male reproductive system. Together with 
the discussion of these effects, their significance to male fertility is also 
debated.