Sufferers of rheumatoid arthritis who take the antirheumatic drug methotrexate increase their risk of developing certain types of cancer, suggests new research.

Rheumatoid arthritis is a chronic inflammatory illness that affects the joints, as well as internal organs such as the eyes, lungs or heart. It affects one in 100 Canadians, according to the Arthritis Society of Canada.

Methotrexate suppresses the immune system, which in rheumatoid arthritis is overactive.

Australian researchers studied 459 rheumatoid arthritis patients — 309 women and 150 men — who started taking methotrexate before June 1986.

After 4,273 person-years of followup, which averaged 9.3 years per patient, they found 87 malignancies.

They discovered that the risk of developing non-Hodgkin's lymphoma in RA patients was five times higher than in the general population.

Patients also had a three-fold increased risk of the deadly skin cancer melanoma and a three-fold increased risk of lung cancer.

Overall, RA patients taking methotrexate were twice as likely as those without the condition to develop cancer.

The researchers state that although previous studies have made links between methotrexate and non-Hodgkin's lymphoma and lung cancer, the melanoma finding was surprising.

"This study is, to our knowledge, the first to report an increased risk of melanoma in patients with RA treated with [methotrexate] compared with the general population," said Rachelle Buchbinder, the study's lead author, in a release.

"Our findings, taken together with other studies investigating the risk of skin cancer in patients with RA, may support a role for regular skin cancer screening for all patients with RA, particularly those receiving immunosuppressive therapy," write the authors.

They believe that immunosuppression may lead to the development of cancer, as the body's defences are weakened.

The study will be published in the June 15 issue of Arthritis Care and Research.

In other arthritis news, the U.S. Food and Drug Administration announced Wednesday that it is investigating whether drugs used to treat rheumatoid arthritis and Crohn's disease in young adults and children are linked to cancers.

These drugs, belonging to a class of medications called TNF (Tumor Necrosis Factor) blockers, include Johnson & Johnson's Remicade, Abbott Laboratories Inc.'s Humira, and Amgen Inc. and Wyeth's Enbrel.

The FDA said it was investigating 30 cancer cases from between 1998 and April 2008. Half of the cases were lymphomas.

The FDA said it would release the findings of its investigation in six months.