Omeprazole
"Have been on 20 mg brand name and OTC omeprazole for 20 years, daily.  Prilosec and Nexium brands have always worked well. However, plain old generic 20 mg omeprazole, which is cheaper, has mixed results. Frequently,  it is not effective so I have to take 2 over a 24 hr period to get the same relief.  With next purchase I return to the brand name for the usual relief. This effect is not isolated, having experienced it many times since the drug went generic. I notice the generic maker is a huge pharmaceutical firm overseas. Who compares and verifies the actual strength of the generic vs. the domestic brand name?"