Abbott Labs ( ABT) got an early New Year's gift: Regulatory clearance to sell its new rheumatoid arthritis drug, Humira. Tuesday afternoon, the Food and Drug Administration approved Humira -- about two months earlier than analysts had expected. With Humira now headed to pharmacy shelves, the long-expected marketing battle with existing treatments from Amgen ( AMGN) and Johnson & Johnson ( JNJ) can finally get started. Humira, also known as D2E7, is administered via injection once every two weeks. That is expected to give the drug a leg up on its rivals. Amgen's Enbrel must be injected twice a week, while Johnson & Johnson's Remicade requires an intravenous infusion. All three drugs work the same way to reduce the swelling and joint tenderness common to sufferers of rheumatoid arthritis. They do so by blocking a molecule called antitumor necrosis factor, which causes inflammation when secreted by cells in the immune system. The three drugs have not been tested against each other, but many analysts believe Humira's convenience will be seen as a big plus by doctors and patients. Last week, Amgen received FDA approval for a new Enbrel manufacturing facility, which is expected to help the company finally resolve its long-standing supply constraints. Amgen has forecast 2003 Enbrel sales in the range of $1.2 billion to $1.4 billion. Humira's entry into the market isn't likely to change that forecast, since its approval was widely anticipated. Abbott Labs shares were up 5% to $39.20 in recent trading. Amgen shares were off nearly 2% to $48.30.