People who repeatedly use certain types of antibiotics may have an increased risk of developing type 2 diabetes, a new study has found.

Antibiotics are used to treat bacterial infections. They cannot treat viral infections. According to US researchers, the overprescribing of antibiotics is already a major issue in healthcare because of the emergence of bacteria that have become resistant to these drugs' effects.

They believe that these latest findings provide yet another reason to reduce unnecessary prescribing.

They assessed the number of antibiotics given to over 200,000 people with diabetes in the UK the year before they were diagnosed with the disease, and compared these to 800,000 people without diabetes.

The study found that participants who were prescribed at least two courses of four different types of antibiotics - penicillins, cephalosporines, quinolones and macrolides - had an increased risk of developing type 2 diabetes. These are used to treat a range of bacterial infections, including respiratory, ear and skin infections.

Furthermore, this risk increased the more antibiotics were prescribed. For example, people who were prescribed between two and five courses of penicillins had an 8% increased risk of developing diabetes. Those prescribed with penicillins more than five times saw their risk of diabetes jump by 23%.

People prescribed two to five courses of quinolones had a 15% increased risk of developing diabetes, while those prescribed more than five courses had a 37% increased risk.

The results stood even when other factors were taken into account, such as obesity and heart disease.

The researchers suggested that antibiotics may alter a person's gut bacteria, increasing their risk of type 2 diabetes.

"Gut bacteria have been suggested to influence the mechanisms behind obesity, insulin resistance and diabetes in both animal and human models. Previous studies have shown that antibiotics can alter the digestive ecosystem.

"While our study does not show cause and effect, we think changing levels and diversity of gut bacteria could explain the link between antibiotics and diabetes risk," they commented.

Interestingly, the researchers found little evidence of a link between antibiotics and type 1 diabetes.

"Over-prescription of antibiotics is already a problem around the world as bacteria become increasingly resistant to their effects. Our findings are important, not only for understanding how diabetes may develop, but as a warning to reduce unnecessary antibiotic treatments that might do more harm than good," they added.

Details of their findings are published in the European Journal of Endocrinology.