Q. How can I remember when I bought certain cosmetics? Are there telltale signs that a product has gone bad?

A. It may seem as if you bought that mascara yesterday. But if it’s giving off a strange odor, the best route is to accept that all good things must come to an end. Makeup expiration dates are decided by each manufacturer, so they vary for different product categories and within each category (for a general guideline on beauty products, see Beauty Products Expiration Dates). The companies determine the dates through a process called stability testing, during which the products are observed through extremes of hot and cold temperatures. “The testing helps the manufacturer to figure out which formulations remain stable under these conditions and to see whether or not the products maintain their aesthetic and functional integrity,” says Mort Westman, president of Westman Associates, a cosmetics and toiletries research-and-development consultancy based in Oak Brook, Illinois.

The Food and Drug Administration only requires expiration dates on cosmetic products with active ingredients, such as acne treatments and sunscreens. If you look at the fine print on the back of the packaging, however, you might see a Period After Opening (PAO) symbol, pictured as a jar with a detached lid with a number and an “M” on it; this serves as another indicator of the product’s anticipated life span. The number is the number of months you should use the product after it has been opened, with the “M” standing for “months.” The clock for an expiration date starts running as soon as a product is made. But for PAOs, start counting from the time you open the product.

If you often lose track of when you bought something, scribble the date of purchase on the tube or jar with a permanent marker, and follow this general rule of thumb: “If the product has a rancid odor or anything growing on the surface, or if it appears significantly different from the day you purchased it, throw it out,” says Westman. “If you don’t, you might not get the performance you want or, worse, you may suffer from irritation. Even if you can’t detect a noticeable change in the product, but it’s past its expiration date, don’t take any chances.” There might be more subtle changes that you can’t see, so err on the side of caution and observe the dates, Westman advises.

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