HOUSTON -

A new drug to treat rheumatoid arthritis is showing promise in large clinical trials.

Rheumatoid arthritis is a painful autoimmune disease that occurs when the immune system attacks its own joints and tissues. There's no cure, but there are medications that often simply treat the pain.

"Some drugs are aimed just to treat the signs and symptoms and some actually change the progression of the disease," explained Dr. Elaine Husni from the Cleveland Clinic.

Falling into the latter category is a new, experimental drug from Pfizer called Tofacitinib. Other disease-altering drugs on the market must be injected, but this drug is taken in pill form.

Doctors said it's important for arthritis patients to have multiple drug options.

"There are a subset that are either not responding fully, or actually not responding at all, and that is the reason we have new drugs coming out," said Husni.

In two new large clinical trials, Tofacitinib was found to be effective and improved physical function in patients. It's also in a totally different class of drugs than pain medications like Vioxx, which is now off the market because of its link to severe heart problems.

So far, doctors said the most common side effects of Tofacitinib have been headache and upper respiratory infections.

Husni explains, "Addressing the longer term issues and experience with the clinician are still going to have to occur before we understand the long-term side effect profile of this drug."