Hi there ~ I understand your concern. It might be helpful to switch you anti acid medication from zintac to a PPI or a proton pump inhibitor like pantoprazole or omeprazole or such with consulting your doctor. Also digene syrup may be helpful for gastritis. You will benefit from taking these medications before meals. It is also helpful to know that the referred pain in your shoulder may be due to having gastric ulcers and may go away if your gastritis is adequately treated. I also think that you should consult a doctor specializing in back problems for adequate relief. I hope this helps. Take care and have a lovely day!