LOS ANGELES (Reuters) - A new drug designed to compete with popular anti-inflammatory pills like Motrin and aspirin, is effective in treating the most common and severe forms of arthritis, researchers said. Researchers said phase III trials of Vioxx, known generically as rofecoxib, showed it was equally effective in relieving pain and inflammation in patients with osteoarthritis as the most commonly prescribed drugs currently available. Phase III trials are typically the last stage before seeking regulatory approval for a drug. Osteoarthritis is the most common form of arthritis. It is caused by the wearing down of cartilage in the joints and often results in patients needing hip or knee replacement surgery. Commonly available drugs such as Motrin, aspirin and Voltaren also work by inhibiting the Cox-2 enzyme, but often cause ulcers and other gastrointestinal side effects by also blocking the Cox-1 enzyme which protects the stomach lining. About 76,000 people are admitted to hospitals annually as a result of side effects from anti-inflammatory drugs with about 7,600 deaths each year attributed to the drugs. A separate phase II trail of Vioxx found it to be effective in treating rheumatoid arthritis, the most severe form of the disease.