Lansoprazole is a proton pump inhibitor (PPI) medication used to treat conditions such as gastroesophageal reflux disease (GERD), peptic ulcers, and Zollinger-Ellison syndrome. It works by reducing the amount of acid produced in the stomach.

Common side effects of lansoprazole include headache, diarrhoea, nausea, abdominal pain, constipation, and gas. It can also interact with certain medications, such as warfarin, so it is important to tell your doctor about all the medications you are taking before starting lansoprazole.

It is important to take lansoprazole exactly as prescribed by your doctor. Do not take more or less of it or take it more frequently than prescribed. 

Further information about the medication:

https://www.nhs.uk/medicines/lansoprazole/