(Reuters) – U.S. advisers opened a two-day meeting on Tuesday to consider whether GlaxoSmithKline’s diabetes drug Avandia is too dangerous to stay on the market.

Regulators in Europe also will meet this month to debate whether the pill should be restricted or even banned.

Documents released on Friday show the experts strongly disagree about the drug’s safety.

Here are some questions and answers about Avandia:

WHAT IS THE PROBLEM WITH THIS DRUG?

Diabetes is a serious chronic illness and is a direct cause of heart disease, but several studies have shown that Avandia, known generically as rosiglitazone, may itself damage the heart. Both Avandia and rival drug Actos, made by Takeda Pharmaceutical Co and known generically as pioglitazone, raise the risk of heart failure.

Two studies published last month showed that Avandia raises the risk of heart attack and stroke compared to Actos, but a third study showed that diabetics who took Avandia had a lower risk of heart attack, stroke or death than patients taking drugs of a different type.

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. The first drug in the class, Rezulin or troglitazone, was pulled from the market in March 2000 after 63 people who took it died from acute liver failure and nearly 40 others needed liver transplants.

WHO DID THE STUDIES?

One of the studies was a “meta-analysis” of 56 trials involving people taking Avandia or other diabetes drugs. Done by longtime Avandia critic Dr. Steven Nissen at the Cleveland Clinic it found those taking Avandia were 28 percent to 39 percent more likely to have a heart attack.

The second study by David Graham and colleagues at the U.S. Food and Drug Administration and the U.S. Centers for Medicare and Medicaid Services found people taking Avandia had 1.25 times the risk of heart failure compared with those taking Actos, 1.27 times the risk of a stroke and 1.14 times the risk of dying.

The third study, paid for by the U.S. National Institutes of Health, found that adding Avandia to the mix of diabetes drugs lowered the risk of heart attack, stroke or death by 28 percent.

WHY IS THIS DRUG STILL ON THE MARKET?

That is just what consumer groups such as Public Citizen and Consumers Union are asking.

But it can be difficult to show that a drug is dangerous, especially in a disease like diabetes, which is already usually complicated with heart disease and other symptoms as well.

More than 700 pages of internal documents released on Friday showed FDA staffers disagree about how to interpret the studies.

Members of the U.S, Congress, including Republican Senator Charles Grassley, Democratic Representative Rosa DeLauro and Senate Finance Committee Chairman Max Baucus, have questioned the FDA’s regulation of Avandia.

A current trial called Tide is designed to directly compare Avandia and Actos.

The FDA has scheduled an advisory panel meeting on the heart safety of Avandia on July 13-14. The European Medicines Agency’s Committee for Medicinal Products for Human Use will discuss the issue in London on July 19-22.

HAVE PEOPLE SUED OVER THIS?

Yes. In May, a lawyer involved in some of the suits said Glaxo had settled with nearly 700 people who said they suffered harm because they took Avandia.

Joseph Zonies, one the lead plaintiffs’ lawyers in the federal case pending in a U.S. District Court, estimated that 3,000 cases have been filed in the federal litigation and says another 4,000 to 6,000 could be filed later.

WHY IS THIS DRUG NECESSARY?

The World Health Organization estimates that 171 million people globally had diabetes in 2000 and projected that number will nearly double by 2030 to 366 million.

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 in the eyes can cause blindness.

While diet and exercise can control diabetes, many people also take prescription drugs.

WHAT OTHER DRUGS ARE THERE?

Diabetics have 12 classes of drugs to choose from.

New drugs include Merck’s Januvia and AstraZeneca and Bristol-Myers Squibb’s Onglyza. Many other drugs are in clinical trials, attacking diabetes with a variety of approaches.

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

(Writing by Maggie Fox; Editing by Lisa Richwine and Tim Dobbyn)