Yes, methotrexate can cause lung problems. The most well-known is methotrexate pneumonitis, a type of lung inflammation that affects roughly 1% to 7% of people taking the drug, though prospective studies tracking patients in real time suggest the actual rate may be closer to the lower end of that range. It can develop at any point during treatment, from days after the first dose to years later, and the symptoms often mimic a respiratory infection, which makes it easy to miss.
How Methotrexate Affects the Lungs
The exact way methotrexate damages lung tissue isn’t fully settled, but two main mechanisms are at play. The first, and most widely supported, is a hypersensitivity reaction. Your immune system essentially overreacts to the drug and triggers inflammation in the lungs, similar to an allergic response. The second mechanism is direct toxicity: methotrexate can injure the thin walls of the air sacs in your lungs, the structures responsible for moving oxygen into your blood. In practice, both processes likely contribute.
There’s also a third, more indirect route. Because methotrexate suppresses part of your immune system, it can leave you vulnerable to repeated viral or other infections that settle in the lungs. This makes diagnosis tricky, because an infection and drug-induced lung inflammation can look very similar on scans and even under a microscope.
Symptoms to Watch For
The classic symptoms of methotrexate-related lung injury are shortness of breath, a dry cough that doesn’t produce mucus, unusual fatigue, and fever. These can come on suddenly or build gradually over days to weeks. The onset can be acute, where symptoms appear rapidly and feel dramatic, or subacute, where breathing slowly gets harder and you might initially chalk it up to being out of shape or fighting a cold.
Because these symptoms overlap with common illnesses, many people don’t immediately connect them to their medication. If you’re on methotrexate and develop a persistent dry cough or notice that activities that were easy before now leave you winded, that’s worth flagging to your doctor promptly.
When It Can Happen
One of the unsettling things about methotrexate pneumonitis is that it doesn’t follow a predictable timeline. Published case reports describe patients developing it within days of starting the drug, while others developed it five or even six years into treatment. There is no safe window after which you can assume you’re in the clear. The condition can appear whether you’re on a low dose for rheumatoid arthritis or a higher dose for another condition.
A large prospective study that followed 223 patients starting methotrexate found only two cases of pneumonitis over the study period, translating to roughly one case for every 192 patient-years of use. That’s reassuring in absolute terms, but it reinforces that vigilance matters for as long as you’re taking the medication.
How It’s Diagnosed
There’s no single blood test or scan that definitively confirms methotrexate pneumonitis. Diagnosis is largely a process of elimination. Your doctor will typically order a CT scan of your chest, which often shows a pattern of hazy, ground-glass patches spread across both lungs, sometimes with fine web-like markings or areas of denser cloudiness. These findings are suggestive but not unique to the condition.
One major challenge is distinguishing methotrexate pneumonitis from an opportunistic lung infection called Pneumocystis pneumonia (PCP), which can also occur in people on immunosuppressive drugs. Both conditions can look similar on imaging. Research comparing the two found that methotrexate pneumonitis tends to produce a specific pattern on CT scans: uniform hazy patches with clear boundaries along the natural divisions of the lung, seen in about 70% of cases. PCP, by contrast, tends to show hazier, more diffuse cloudiness without those sharp boundaries. Still, the overlap is enough that doctors sometimes need additional testing to tell them apart.
Pulmonary Function Tests
You might wonder whether routine breathing tests can catch the problem early. Unfortunately, research has shown that standard pulmonary function tests cannot predict methotrexate pneumonitis before symptoms appear. One study of 118 patients on long-term, low-dose methotrexate did find small, statistically significant declines in certain lung measurements over time, including a 5% drop in the volume of air they could forcefully exhale in one second and a nearly 5% drop in how efficiently their lungs transferred gases. But these subtle changes weren’t useful as early warning signs for the more serious pneumonitis. In short, normal breathing test results don’t guarantee your lungs are unaffected, and abnormal results don’t necessarily mean you have pneumonitis.
What Happens if You Develop It
The first and most important step is stopping methotrexate. In many cases, the lung inflammation begins to improve once the drug is out of your system. For more significant cases, corticosteroids are used to calm the immune response and speed recovery. Published case reports describe successful treatment with corticosteroids and supportive care, including supplemental oxygen if blood oxygen levels drop.
Most people who are diagnosed and treated promptly recover well. However, the condition isn’t without risk. In rare cases, particularly when diagnosis is delayed, the inflammation can progress to permanent scarring of the lung tissue (pulmonary fibrosis), which causes lasting breathing difficulties. The key factor in outcomes is how quickly the drug is stopped and treatment begins.
If you’ve had methotrexate pneumonitis, restarting the drug is generally avoided. The reaction is likely to recur, and a second episode can be more severe. Your doctor will typically switch you to a different medication for whatever condition the methotrexate was treating.
Separating Drug Effects From Disease Effects
This is an important nuance for people taking methotrexate for rheumatoid arthritis or other autoimmune conditions. Many of these diseases can independently cause lung problems. Rheumatoid arthritis, for example, is associated with its own forms of lung inflammation and scarring. So if you develop breathing symptoms while on methotrexate, the cause could be the drug, the underlying disease, an infection enabled by immune suppression, or some combination. Sorting this out usually requires imaging, lab work, and sometimes a procedure to sample fluid or tissue from the lungs.
This overlap is exactly why new or worsening respiratory symptoms on methotrexate should never be dismissed or self-diagnosed. The treatments for each possible cause are different, and getting the right diagnosis matters.