There has been some controversy about AstraZenecas behaviour in creating, patenting, and marketing the drug. Esomeprazoles successful predecessor, omeprazole, is a mixture of two mirror-imaged molecules (esomeprazole which is the S-enantiomer, and R-omeprazole); critics said the company was trying to "evergreen" its omeprazole patent by patenting the pure esomeprazole and aggressively marketing to doctors that it is more effective than the mixture.