Oxford Pharmascience Ltd (the Sponsor) is developing a new naproxen tablet formulation (the study drug, OXP005), for the potential treatment of rheumatic and painful conditions. The study will compare the study drug to an already marketed formulation of prescription strength naproxen (Naprosyn® the reference product) by looking at the relative impact of both products on gastroduodenal (GD) irritation by performing a specialist procedure called an endoscopy (or more specifically, a gastroscopy). The safety and tolerability of the study drug will also be assessed.