When the British began importing large amounts of the drug, the Chinese began using opium for its mind-changing effect. More and more people grew addicted to opium. So, the British were able to export more and more opium. By selling this drug, the British slowly began to make more money on their exports to China than they spent on their imports of Chinese goods. British exports of opium to China increased greatly. They went from an estimated 15 tons in 1730, to 75 tons in 1773. Opium was shipped in "chests". Every chest had 67 kilograms (140 pounds) of opium inside.