A recent study shows that glitazones (thiazolidinediones), a class of drugs used to treat diabetes, increase the risk of fluid buildup in the eye, a condition known as diabetic macular edema. Left untreated, this can cause blurred vision and vision loss.

Macular edema is caused when excess fluid leaks into the retina, causing swelling. The condition is treatable, usually by laser surgery. All diabetics have an increased risk of developing macular edema; those who take glitazones appear to have an even higher risk. In the current study, the researchers found that taking a glitazone increased a person's risk of developing macular edema by 60%. This is not the first study to suggest this association, but it is the largest to date.

Glitazones are a relatively new class of diabetes drug. They work by increasing the sensitivity of muscle, fat and liver tissue to insulin. This causes these tissues to take up more glucose, lowering the amount of glucose in the blood. The two glitazones currently being prescribed are pioglitazone (Actos®) and rosiglitazone (Avandia®). These drugs are sometimes combined with other diabetes drugs, and the combined medications are sold under various different names.

Glitazones have always been a controversial treatment for diabetes. The first of them, troglitazone, was introduced in the late 1990s and withdrawn from the market in 2000, when it was found to cause serious liver problems. Rosiglitazone was approved by the FDA in 1999; pioglitazone at a later date. Various studies have linked both of these drugs to an increased risk of bone fracture and congestive heart failure. One study linked rosiglitazone to an increased risk of heart attack and death from cardiovascular disease. Overall, these studies have raised more concerns over the safety and effectiveness of rosiglitazone than pioglitazone, though neither gets an entirely clean bill of health.

There is disagreement among doctors as to whether the benefits of these drugs outweigh their side effects or not.

All this has to leave glitazone users scratching their heads. Anyone wondering why a glitazone was prescribed to them should speak with their doctor about it.

The current study looked at records of 143,000 U.S. patients treated with diabetes medications between 2002 and 2006. It focused on the 996 new cases of macular edema diagnosed by eye examinations in 2006. Simply looking at how many glitazone users developed macular edema showed these users to be 160% more likely to develop macular edema, a risk somewhere between double and triple of non-users. However, taking into account other medical factors that are known to cause macular edema, the researchers concluded that the actual increase in macular edema cases due to glitazones was only 60%.

The results of this study were published in the April 2009 issue of the Journal of Ophthalmology.