Updated from 9:58 am. EDTThe weakened economy is causing rheumatoid arthritis patients to put off or delay treatment with expensive injectable drugs and pressuring the leading market players Amgen ( AMGN), Johnson & Johnson ( JNJ) and Abbott Labs ( ABT). A new survey of 61 U.S. rheumatologists released Wednesday by J.P. Morgan revealed that 68% said the economy was having a negative impact on the prescription of drugs known as anti-TNFs for rheumatoid arthritis in the past three months. Explaining the slowdown, 58% of doctors said patients were finding it more difficult to get their insurance companies to pay for anti-TNF therapy, while 79% of doctors said patients were having a tough time paying out-of-pocket costs for the drugs, according to the survey. "Given the high cost of therapy ($17,000 to $25,000 per year), this isn't a surprise, but it is the first time that the economic downturn has had an impact on market growth of a broad class like the anti-TNFs with $16 billion in 2008 sales," wrote J.P. Morgan analyst Geoff Meacham, in a Wednesday research note discussing the survey results. Investors are watching the downtrend in rheumatoid arthritis drug sales with concern. Recent weakness in Amgen's stock price is attributable, in part, to slacking first-quarter prescription data and management's cautious comments about Enbrel, the company's anti-TNF drug, which accounted for $3.6 billion, or 23%, of Amgen's revenue last year. Analysts expect first-quarter Enbrel sales of $918 million, essentially flat with fourth-quarter sales and down a bit more than 3% year over year.