Is vermouth supposed to be refrigerated?

Yes — vermouth is a fortified wine and oxidizes once opened. Refrigerate after opening and use within 30 to 60 days for best flavor.

Vermouth (both dry and sweet) is wine fortified with neutral spirit and infused with botanicals. Like any wine, it oxidizes after opening; unlike spirits at 40 percent ABV, vermouth at 16 to 18 percent ABV does not have the alcohol concentration to fully protect itself. Refrigeration slows oxidation roughly fivefold and is the standard professional bar practice. Once opened, dry vermouth holds its flavor for about 30 days, sweet vermouth for about 45 to 60 days, before noticeably flat, dull notes appear. Tell-tale signs of oxidized vermouth: a sharp, vinegary edge on the finish; the loss of bright floral or spice notes; a slightly darker color. If a Martini suddenly tastes harsh and a Manhattan suddenly tastes flat, replace the vermouth bottle. Pre-Prohibition home bars left vermouth at room temperature because the bottle was emptied within a week; modern home bars cannot match that pace.