To me, the main impact of this war was that it acted as a wedge to open China up. As China was forced to open up more and more to the West (and eventually Japan) it lost its sovereignty and its whole society was undermined.
The actual immediate impacts of the war were not that great. Some opium came into China. Some ports were opened up and some missionaries were allowed in. But in the long run, this was just the start. From there, the British and other countries started to demand more and more access until they practically destroyed the soveriegn power of the Chinese government.
By the 19th century, a new type of barbarian, the Europeans, threatened China. Initial confrontations arose over the British plan to export opium from India to China in order to improve the European balance of trade. The Qing government recognized the threat to both its economy and its society posed by unlimited importation of opium. In the 1830s, the Qing emperor appointed Lin Zexu, a renowned bureaucrat, to stamp out the opium trade. Lin blockaded Canton and confiscated European opium supplies. British merchants demanded that their government intervene to protect investments. In 1839, the British routed the Chinese junks in the first stages of the Opium War. When the British sent a military force ashore, the Qing emperor sued for peace. By the 1890s, 90 Chinese ports were open to European, Japanese, and American merchants. Britain, France, Germany, and Russia actually leased certain ports and their hinterlands. Trade passed increasingly into the hands of the non-Chinese, and the Qing court was forced to accept European diplomats.
The establishment of the Canton trading system in China saw a dramatic rise in the import of opium via the British East India Company. The widespread smoking and trading of opium not only threatened public health, but also damaged the national economy as Chinese silver, which served as payments for British opium imports, flowed out of the country rapidly. In 1838, the Qing Emperor, Daoguang, decided to take a hardline stance against the opium trade and appointed a Fujian scholar, Lin Ze Xu, to end the practice of opium trading in Canton. Lin initiated a campaign against opium trading and smoking in the city, which was carried out firmly and successfully by his subordinates. The British, angered at the loss of profits from the burning of opium, responded militarily by sending in four British fleets to blockade the entrance to the Canton harbour, leading to the outbreak of the Opium War. Severe fighting between the British troops and the Chinese forces lasted for two years, ending with the defeat of the Qing army. The victory of the West was a wake-up-call for China - the Manchu troops were clearly no match for the new and powerful weapons that the British had. The capture of Nanjing in 1842 prompted the Qing government to quickly sue for peace, which led to the signing of the Treaty of Nanjing, and ended China’s first clash with the West. The treaty fundamentally altered the structure of Qing relations with foreign powers, and paved the way for the signing of a series of unequal treaties with other Western states.