Drinking in Japan can be overwhelming. The good news for visitors fond of their booze: Japan is pretty much a drinker's paradise and always has been. It seems even as far back as the 3rd century there has been a very strong drinking culture in Japan. Most establishments are about the size of a walk-in closet and seat about eight people. Bars open and close as and when they please, and no license is needed to operate one. The primary drinks you'll find people enjoying are beer, wine, and a cocktail affectionately named the highball.
Since many Japanese are unable to drink too much (many lack the necessary enzyme to break down alcohol and turn bright crimson), the Highball (haibōru, ハイボール) is the closest to an american cocktail that you will experience in Japan. It's generally a single serving of whiskey and three parts mixer (soda, ginger-ale, etc) Many other cocktails on the menu in Japan are mostly cordials & juice, and I would recommend avoiding them as they are more sugar than anything else.
Many Japanese feel that after work parties are an important way to enhance relationships, these parties are aptly named nomikai (nomu meaning drinking and kai meaning meeting or party) A common saying in Japan is, “if you want to work your way up the corporate ladder you have to drink”. This was how many older generation workers established relationships and considered this the normal way of doing business.
It is considered rude to pour a drink for oneself. In a gesture of hospitality, your drinking partner will pour your drink. You then reciprocate by pouring theirs. Keep watch to make sure the people sitting around you have something in their glass at all times. If you aren’t paying attention, your partners will invariably keep insisting on filling your glass, even if you don’t want to continue drinking. This takes some getting used to, especially when you are enjoying the conversation. (I learned this lesson the hard way.) If you’re not a heavy drinker, keep your glass almost full. an important point of etiquette which differs from Western business culture is that it is considered acceptable to become drunk at a nomikai. In the same vein, things said and done under such circumstances are not taken seriously, but are forgiven or ignored upon return to the workplace.