Visiting a Japanese drugstore is an 'adventure', even for someone like me who is a nurse and can figure things out in Japanese. My prior experience with visiting a drugstore in Japan is rare because my mother was a pharmacist and she had access to all the awesome remedies! Since she passed away, though, we're sort of on our own. Best just to describe your symptoms to the pharmacists behind the counter and ask for a recommendation. Most of them can understand some English.
Here's some helpful information from the blog, Surviving in Japan Without Much Japanese: 7 words to know when you have a cold (in Japan).
After describing our symptoms of a cough/phlegm, we were given this medicine from behind the counter. Fortunately, many of the active ingredients (except for the eastern medicine components) are listed in their generic names, which are universal (except for acetaminophen versus paracetamol, etc.), as written previously in the post, Foreign Equivalents of Over-the-Counter Meds.