Get emergency medical help if you have signs of an allergic reaction ro lansoprazole: hives; difficulty breathing; swelling of your face, lips, tongue, or throat.
Call your doctor at once if you have:
severe stomach pain, diarrhea that is watery or bloody;
new or unusual pain in your wrist, back, hip, or thigh;
a seizure (convulsions);
kidney problems - fever, nausea, little or no urination, blood in your urine, swelling, rapid weight gain;
low magnesium - dizziness, fast or irregular heart rate, tremors (shaking) or jerking muscle movements, feeling jittery, muscle cramps, muscle spasms in your hands and feet, cough or choking feeling; or
new or worsening symptoms of lupus - joint pain, and a skin rash on your cheeks or arms that worsens in sunlight.
Taking lansoprazole long-term may cause you to develop stomach growths called fundic gland polyps. Talk with your doctor about this risk.
If you use lansoprazole for longer than 3 years, you could develop a vitamin B-12 deficiency. Talk to your doctor about how to manage this condition if you develop it.
Common lansoprazole side effects may include:
nausea, stomach pain;
diarrhea, constipation; or
headache.
This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.
 Lansoprazole side effects (more detail)
