A look at blockbuster anti-TNF drugs and the visibility of entrenched brands including AbbVie’s Humira (adalimumab) and Johnson & Johnson’s Remicade (infliximab), could make it seem like Biogen Idec‘s effort to break into the category will be daunting.

However, a new study indicates why the market could make room for another competitor: rheumatoid arthritis patients tend to abandon their first anti-TNF treatment at fairly high rates.

Swedish researchers found that most RA patients dropped their first anti-necrosis factor medications because they didn’t work, or appeared to lose their effectiveness. Although the abandonment rates differed slightly by geography, the researchers found that Swedish patients abandoned Remicade at higher rates than Humira, and dropped Humira more often than Amgen’s Enbrel (etanercept) within the first 12 months.

American patients were another matter. Researchers said US patients tended to stick with Remicade a bit longer, but noted that this can probably be chalked up to several factors. Among them: Americans tend to get higher doses than their European counterparts, they usually receive the drug earlier in their RA diagnosis (when symptoms are less intense). There is also the Medicare factor — the government program reimburses the price of the Remicade infusion, but not DIY injectibles Humira or Enbrel.

Researchers noted that Swedish patients don’t tangle with a similar variable because its government-supported healthcare system does not restrict access to biologics or have a step-therapy process that requires a prescription progression before patients can receive a specific RA medication.

As for just how quickly patients leave Remicade, researchers found that 50% of users stopped within 2.6 years, while 50% of Humira-first patients stayed with the drug for five years. At the end of five years, 38% of Remicade-first RA patients continued with the infusion, versus 50% of Humira-first RA patients, and 55% of Enbrel-first recipients.

Each of these drugs has a laundry list of overlapping indications — including rheumatoid arthritis, plaque psoriasis and psoriatic arthritis — but the overall oomph they provide is clear. In AbbVie’s case, Humira earned enough across its indications ($10.7 billion in 2013) to launch its split from Abbott. Similarly,  Remicade’s multiple indications added $6.7 billion to JNJ’s 2013 sales, while Enbrel boosted Amgen’s 2013 financials by $4.6 billion.