See the DrugPatentWatch profile for methotrexate

Methotrexate, a medication commonly used to treat conditions such as rheumatoid arthritis, psoriasis, and certain types of cancer, can potentially interact with other medications, which may result in decreased or increased efficacy of the concomitantly administered drugs [1].

Methotrexate can decrease the effectiveness of some medications. For instance, methotrexate can reduce the efficacy of folic acid and folinic acid, which are sometimes used to reduce the side effects of methotrexate itself [1]. This interaction occurs because methotrexate and folic acid/folinic acid compete for the same transport protein, reducing the absorption of folic acid/folinic acid [1].

On the other hand, methotrexate can also increase the efficacy of some medications. For example, methotrexate can enhance the effects of certain chemotherapeutic agents, such as fluorouracil and capecitabine, which are used to treat various types of cancer [1]. This interaction is attributed to methotrexate's ability to inhibit dihydrofolate reductase, an enzyme that plays a crucial role in DNA synthesis [1]. By inhibiting this enzyme, methotrexate can potentiate the cytotoxic effects of fluorouracil and capecitabine [1].

It is important to note that drug interactions can be complex and may depend on various factors, including the dosage, frequency, and route of administration of the medications involved, as well as the individual patient's health status [1]. Therefore, it is essential to consult with a healthcare provider or pharmacist before starting, stopping, or changing any medications, including methotrexate, to ensure safe and effective treatment [1].

Sources:
[1] DrugPatentWatch. "Methotrexate Drug Interactions." DrugPatentWatch.com. <https://www.drugpatentwatch.com/drugs/methotrexate/interactions>.