Has Nexium (esomeprazole) been known to cause people to gain weight?

No, but... Although not reported as a common side effect, if someone has not been eating much due to pain from an ulcer or esophagitis, then improvement in symptoms that comes with treatment of these ailments could result in improvement in appetite and subsequent weight gain. If food intake and activity have not changed, there should be no weight gain from Nexium (esomeprazole) or similar medications.