Diarrhea, abdominal pain, or headache may occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly. Remember that this medication has been prescribed because your doctor has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects. Tell your doctor right away if you have any serious side effects, including: symptoms of a low magnesium blood level (such as muscle spasms, irregular heartbeat, seizures), signs of lupus (such as rash on nose and cheeks, new or worsening joint pain), signs of kidney problems (such as change in the amount of urine, pink/bloody urine). This medication may rarely cause a severe intestinal condition due to a bacteria called C. difficile. This condition may occur during treatment or weeks to months after treatment has stopped. Tell your doctor right away if you develop: diarrhea that doesn't stop, abdominal or stomach pain/cramping, blood/mucus in your stool. If you have these symptoms, do not use anti-diarrhea or opioid products because they may make symptoms worse. Rarely, proton pump inhibitors (such as lansoprazole) have caused vitamin B-12 deficiency. The risk is increased if they are taken every day for a long time (3 years or longer). Tell your doctor right away if you develop symptoms of vitamin B-12 deficiency (such as unusual weakness, sore tongue, or numbness/tingling of the hands/feet). A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: fever, swollen lymph nodes, rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing. This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist. In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch. In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

A product that may interact with this drug is: methotrexate (especially high-dose treatment). Some products need stomach acid so that the body can absorb them properly. Lansoprazole decreases stomach acid, so it may change how well these products work. Some affected products include ampicillin, atazanavir, erlotinib, levoketoconazole, nelfinavir, pazopanib, rilpivirine, sparsentan, certain azole antifungals (itraconazole, ketoconazole, posaconazole), among others. Lansoprazole is very similar to dexlansoprazole. Do not use medications containing dexlansoprazole while using lansoprazole. This medication may interfere with certain lab tests, possibly causing false test results. Make sure lab personnel and all your doctors know you use this drug.