Originally Posted by
ksandness
Other Japanese have had their doctors tell them to stop smoking and/or drinking. This is called dokutaa sutoppu "doctor stop" in Japanese English.
This. If you decline an offer to drink alcohol and get any pushback whatever, just say "doctor stop" in English and they will understand and not push the matter further. And, it's roughly true in the OP's situation, assuming some sort of medical professional was involved in his original decision to stop drinking.
Regarding alternate beverage choices in the home, others have given good advice. Don't assume that something commonly found in an American home will be there in Japan, whether it be cola, milk, or anything else. The first time I stayed in a Japanese home, they asked me what I normally drink with dinner, probably expecting the answer to be beer or wine. When I said "Diet Coke," they were surprised. Unbeknownst to me, a family member was immediately dispatched to go to the store to get some. Nothing was said until they were back with it in the grocery store bag. I was embarrassed at having made them go to all the trouble.