According to an article in the New York Times, medicine cabinets may not be the best location for storing vitamins.  This year, food scientists at Purdue University published a study that found signs of degradation in vitamin C supplements stored above 80 percent humidity, which can be produced by a hot shower.  Similar effects have also been found on thiamin, vitamin B6 and other water-soluble nutritional products in moisture-rich environments.  

In a process called deliquescence, humid environments reduce the potency of vitamins and other supplements to almost nothing.  Therefore, bathrooms and kitchens, which are both known to have spikes in humidity, are not the best surroundings for our supplements.  Research suggests storing vitamins in a cool, dry area such as a bedroom.  The article also recommends throwing away any vitamins that develop softness or dark spots because they may be already degrading. 

Read New York Times article>>