According to Suicide Wiki
(
web.archive.org
)
you should stop taking omeprazole before your attempt.
"Proton pump inhibitors (PPIs): If you are taking PPIs (e.g., esomeprazole, lansoprazole, omeprazole, pantoprazole, and rabeprazole) for gastric ulcers, you should stop taking them 2 weeks prior your attempt. See the Antacid section for more information."