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You have not written which Antibiotics you have taken. Penicillin related antibiotics can interact with Methotrexate
( Methadextrate), and increase the level of Methotrexate in blood. Increase level of Methotrexate can cause side effects like decrease in White blood cells, which can cause sores in mouth. Denture can irritate the sore and can bleed.
You must go and see a Doctor. He or she would examine you and may you to do blood test
tor WBC count
. If it is low, you may have to skip Methotrexate this week.
In future, before taking any medication , you must inform your Doctor that you are on Methotrexate.