Many people take 40 milligrams of omeprazole for various medical conditions, and the side effects vary widely. Some people experience side effects such as drowsiness, muscle spasms, constipation, dizziness and gas, according to the Mayo Clinic. Omeprazole should be taken only as prescribed by a doctor.
There are a multitude of possible side effects related to oral omeprazole ingestion, many of which are rare or less common. Omeprazole is used to treat stomach ulcers, dyspepsia, upper gastrointestinal tract bleeding, GERD and too much acid production in the stomach. A 20 to 40 milligram dose is often coupled with clarithromycin or clarithromycin plus amoxicillin for ulcers with H. Pylori, according to the Mayo Clinic.