Why did the British introduce monopoly over the trade of opium?

In the late 18th century, the English East India Company was buying tea and silk
from China for sale in England. Tea became a popular drink and by 1830, 30 million pounds tea was being imported into England. The tea could be bought only by paying in silver coins or bullion because the Manchus rulers of China were unwilling to allow the entry of foreign goods. Merchants, therefore, looked for ways to stop the loss of silver. They searched for a commodity they could sell in China, something they could persuade the Chinese to buy. And that commodity was opium which was introduced by the Portuguese into China. While the English cultivated a
taste for Chinese tea, the Chinese got addicted to opium. So under these circumstances, the British had no option except to have monopoly over the trade of opium.