I started having abdominal pain in May and my GP put me on 300 mg of ranitidine daily for what he suspected was GERD or an ulcer. I took it for about two weeks with improvement in the pain, but it caused horrendous brain fog and fatigue during the day even though I took my dose at bedtime. I stopped it and within 24 hours, I was flat out with a flare of POTS. It can't be a coincidence. I went from almost normal functioning (driving, doing all errands independently, gardening, housework, etc.) to being almost completely bedbound. I tried taking ranitidine again with some improvement in my POTS symptoms but the nearly unbearable fatigue and palpitations from the medication were affecting me as badly as the POTS symptoms so I stopped it again. I tried a reduced dose of 75 mg with no success. I've been off the medication now for a week and my POTS symptoms are as bad as they've ever been. I can be up and moving around for 10 minutes at the most. When I stand, my heart rate immediately rockets into the 140s and 150s when before it was hanging in there around the 90s to 100s. I fainted yesterday for the first time in my life while sitting up and eating lunch. This stinks. I've parted ways with my GP because he became very angry at the idea that I was having side effects or withdrawal from the ranitidine. It's a well-tolerated drug, apparently, so I shouldn't be having any problems, and if I am, I'm imagining them. Whatever. I know many people with mastocytosis, etc. take ranitidine and see improvement in their symptoms. Has anyone ever had a bad reaction like I have? I'm actually beginning to question myself even though I have a long history of drug intolerance.