So usually Wikipedia is a terrible source but in this case the data seems reliable.

The decade of the 1830s witnessed a rapid rise in opium trade, and by 1838, just before the First Opium War, it had climbed to 40,000 chests. The rise continued on after the Treaty of Nanking (1842) that concluded the war. By 1858 annual imports had risen to 70,000 chests (4,480 long tons (4,550 t)), approximately equivalent to global production of opium for the decade surrounding the year 2000.

A couple of notes: these figures come from work by economic historian Michael Greenberg, compiled in 1951 from British merchant records. JK Fairbank considers them fairly reliable and Jonathan Spence also considers them reliable enough to reprint in his fairly authoritative Search for Modern China, including in the most recent editions. As far as I know these are the best estimates we have.

See Britsh Trade and the Opening of China for the original work.