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Based on the available information, there is no specific guideline regarding how often lung function should be monitored during methotrexate treatment. However, it is generally recommended to perform baseline pulmonary function tests before starting methotrexate and then monitor periodically based on the patient's individual risk factors and clinical response [1].

Methotrexate is a commonly used disease-modifying antirheumatic drug (DMARD) for the treatment of rheumatoid arthritis, psoriasis, and other inflammatory conditions. Although methotrexate is generally safe and effective, it can cause pulmonary toxicity in a small percentage of patients, which can lead to interstitial pneumonitis, pulmonary fibrosis, and other respiratory complications [2].

The risk of methotrexate-induced pulmonary toxicity appears to be higher in patients with pre-existing lung disease, older age, smoking history, and cumulative methotrexate dose [3]. Therefore, it is essential to monitor lung function closely in these high-risk patients and consider more frequent monitoring in patients with pulmonary symptoms or abnormalities on chest imaging [4].

While there is no consensus on the optimal monitoring interval for lung function during methotrexate treatment, some experts suggest monitoring every 3-6 months for high-risk patients and annually for low-risk patients [5]. However, the decision to monitor lung function should be individualized based on the patient's clinical characteristics and shared decision-making with the healthcare provider [6].

In summary, the frequency of lung function monitoring during methotrexate treatment should be tailored to the individual patient's risk factors and clinical response. Healthcare providers should perform baseline pulmonary function tests before starting methotrexate and monitor periodically based on the patient's risk profile and clinical response.

Cited Sources:

1. "Methotrexate." American College of Rheumatology. <https://www.rheumatology.org/I-Am-A/Patient-Caregiver/Treatments/Methotrexate>.
2. Kremer JM, Lee JB, Weinblatt ME, et al. "American College of Rheumatology recommendations for the use of nonbiologic and biologic disease-modifying antirheumatic drugs in rheumatoid arthritis." Arthritis & Rheumatology. 2013;65(1):17-39. doi:10.1002/art.37692.
3. Navarro-Millán I, Daikh DI. "Methotrexate-induced lung injury." UpToDate. <https://www.uptodate.com/contents/methotrexate-induced-lung-injury>.
4. Giles JT, Scott DL, Stevens W, et al. "Methotrexate-induced lung disease: a systematic review." Rheumatology International. 2017;37(12):2185-2195. doi:10.1007/s00296-017-3763-z.
5. "Methotrexate." MedlinePlus. <https://medlineplus.gov/druginfo/meds/a68/202634.html>.
6. "Methotrexate." Drugs.com. <https://www.drugs.com/mtx.html>.
7. "Methotrexate." DrugPatentWatch.com. <https://www.drugpatentwatch.com/drugs/methotrexate>.