Question

why was the opium war called so and what were its major consequences

Answer 1

Opium war occurred between the British and the Chinese . This grew due to the illegal trading by the British of opium in Chinese borders and thus it was called opium war . 
One of its consequence was that Opium was being traded in large scale all over China .  

Answer 2

opium war :-

FOUGHT BETWEEN - British and China

CAUSE - due to illegal selling of opium in the China border

CINSEQUENCE - large scale opium was sold in china