European and U.S. drugs regulators will decide on Thursday whether GlaxoSmithKline Plc's diabetes drug Avandia should be pulled from the market due to concerns about heart risks.

Here are some questions and answers about Avandia:

What's the problem with this drug?

Both Avandia, known generically as rosiglitazone, and rival drug Actos, or pioglitazone, made by Takeda Pharmaceutical Co Ltd, are known to raise the risk of heart failure.

But Glaxo's drug has also been linked to an increased risk of heart attacks, following publication of a study in 2007 by Dr. Steve Nissen of the U.S. Cleveland Clinic, whose findings prompted the latest safety concerns.

In an update to his original 2007 study in June this year, Nissen said a "meta-analysis" of 56 trials involving 35,000 people taking diabetes drugs showed those taking Avandia were between 28 percent and 39 percent more likely to have a heart attack.

Debate about Avandia's safety has raged since 2007. Other research has provided conflicting results and Glaxo says the overall evidence does not show Avandia increases the chances of a heart attack.

Avandia is a member of a drug class called thiazolidinediones or glitazones. They affect a gene called PPAR-gamma and help the body use insulin more effectively, but also have other, broader effects on cells.

Actos is now the subject of a separate U.S. safety review into a possible link to bladder cancer.

How important is Avandia to Glaxco?

Avandia was once Glaxo's second-biggest drug, with worldwide sales peaking at $3 billion in 2006. The drug's sales have plunged since 2007, however, and totaled $1.2 billion in 2009, equivalent to 2.7 percent of Glaxo's group sales. Consensus analyst forecasts compiled by Thomson Reuters suggest the figure will dwindle to around $350 million by 2014.

The drug is set to lose patent protection in the all-important U.S. market in 2012, so sales were always expected to fall sharply within the next couple of years.

Have people sued over Avandia?

Yes. In the United States there have been thousands of cases of people claiming damages after taking the drug. In July, Glaxo said it was taking a legal charge of 1.57 billion pounds ($256 billion) after settling the "substantial majority" of claims relating to Avandia.

The charge also covered other long-standing legal cases, including an investigation into the drugmaker's former factory at Cidra in Puerto Rico, and anti-trust and product liability litigation over antidepressant Paxil.

Why are diabetes drugs important?

The World Health Organization estimates that 171 million people globally had diabetes in 2000 and predicts that number will nearly double by 2030 to 366 million. Most sufferers have Type 2 diabetes, the kind linked to bad diet and lack of exercise.

Diabetes raises the risk of heart disease, stroke, kidney failure and other illnesses. Uncontrolled blood sugar levels can damage the blood vessels and patients can lose toes, feet and legs to diabetes, while kidneys can fail and damage to the eyes can cause blindness.

What other drugs are there?

Diabetics have around a dozen classes of drugs to choose from.

New drugs include Merck & Co Inc's Januvia as well as AstraZeneca Plc and Bristol-Myers Squibb Co's Onglyza. Many other drugs are in clinical trials, attacking diabetes with a variety of approaches. One promising new class of medicines is so-called GLP-1 drugs, such as Eli Lilly and Co and Amylin Pharmaceuticals Inc's Byetta, and Novo Nordisk A/S's Victoza.

Older drugs such as metformin and a class known as sulfonylureas are available generically and can also help lower blood sugar.