Methotrexate is a medication used to treat certain types of cancer, autoimmune diseases, and ectopic pregnancies. According to the Drug Patent Watch website [1], Methotrexate is usually taken once a week, and the dosage may vary depending on the patient's condition. Elderly patients may require a lower dosage due to age-related changes in the body's metabolism.

A study published in the Journal of Rheumatology [2] investigated the effects of methotrexate on elderly patients with rheumatoid arthritis. The study found that methotrexate was an effective treatment for rheumatoid arthritis in elderly patients, with a low incidence of side effects. The study recommended starting with a low dosage and gradually increasing it until the desired effect is achieved.

Another study published in the British Medical Journal [3] analyzed the use of methotrexate in elderly patients with psoriasis. The study found that methotrexate was an effective treatment for psoriasis in elderly patients, but caution should be taken due to the increased risk of toxicity in this age group. The study recommended starting with a low dosage and monitoring the patient closely for any adverse effects.

In conclusion, the recommended frequency of methotrexate for elderly patients depends on the patient's condition and should be determined by a healthcare professional. Generally, a lower dosage is recommended due to age-related changes in metabolism. It is recommended to start with a low dosage and gradually increase it until the desired effect is achieved while monitoring the patient closely for any adverse effects.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/METHOTREXATE
[2] https://pubmed.ncbi.nlm.nih.gov/8972245/
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1741826/