Arava (Leflunomide) is a common first line DMARD used in treating RA, especially if MTX is not tolerated or is ineffective. During the early days of its introduction in 2003, concerns were raised about its possible role in causing lung inflammation, stemming from several reports from Japan, with 4 deaths even.
RA is known to be associated with lung inflammation, so it’s often difficult to discern whether the pneumonitis, when it occurs, is due to the RA or the drugs used to treat it, like MTX.
This recent comprehensive review and metaanalysis of large randomised controlled trials (the gold standard in medical research) not only did not show any increased risk for Arava users, but conversely a protective benefit instead.