Interaction between Oseltamivir and Methotrexate
Moderate Excretion

ID DDInter1350 and DDInter1174
Interaction Oseltamivir may inhibit the renal elimination of methotrexate. The pharmacologic effect and toxicity of methotrexate may be increased, especially in patients receiving high-dose methotrexate. The mechanism is competition for renal tubular secretion.
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Management If this combination must be used, European drug product labeling recommends that patients should be monitored for signs and symptoms of methotrexate toxicity; e.g., bone marrow suppression, hepatotoxicity, and nephrotoxicity. Patients should be advised to promptly report symptoms including fever, chills, sore throat, bruising, bleeding, stomatitis, malaise, shortness of breath, lower extremity edema, jaundice, or change in stool or urine color to their physician.
References
Alternative for Oseltamivir J05A
Raltegravir
Doravirine
Delavirdine
Enfuvirtide
Letermovir
Cobicistat
Amprenavir
Etravirine
Baloxavir marboxil
Boceprevir
Elbasvir
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Alternative for Methotrexate L04A
Alefacept
Muromonab
Canakinumab
Certolizumab pegol
Cladribine
Efalizumab
Antithymocyte immunoglobulin (rabbit)
Secukinumab
Brodalumab
Natalizumab
Azathioprine
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Potential Metabolism Interactions

Substrate-Substrate Interaction：If more than one drug is metabolized by the same CYP, it is possible that its metabolism is inhibited because of the competition between the drugs. That means, it can be useful to lower the dosage of the drugs in the drug-cocktail because they remain longer in the organism than in monotherapy.
Inhibitor-Inhibitor Interaction：Combining two or more inhibitors of one CYP, should be compensated by lowering the dosage of these drugs because the metabolism is reduced and the drugs remain longer in the organism than in monotherapy. Not adapting the dosage bears the risk of even more side effects.
Inhibitor-Substrate Interaction：Combining drugs that have inhibitory effect and are substrates of one particular CYP, should be compensated by lowering the dosage. They rest longer in the organism than in monotherapy. Not adapting the dosage bears the risk of even more side effects.