"Just remember, these are voluntary, you don't have to do these, you can still shop at stores without being part of the loyalty program," Gill of Consumer Reports said.
CVS, Walgreen and Rite Aid all say that they do not sell the data they collect from their loyalty program members.
4. What do the companies get out of all of this?
The cards give companies loads of information to help them drive business to their stores, stock shelves and tailor marketing.
Jonas, an ExtraCare member, receives weekly emails from CVS offering weekend deals. That's a slower time for drugstores because most customers visit during the week to fill a prescription after seeing their doctor.
ExtraCare data help CVS executives figure out where to place items in its stores and what items to offer, said Rob Price, the company's chief marketing officer. For instance, if the company sees that its regular customers are buying several bottles of a shampoo at one size, it might start offering a bigger bottle.
CVS has years of data from its loyalty program. It can tell not only what customers are buying, but which ones like sales that feature a percentage off the price and which want deals that advertise the dollar-value discount, Price said.
These loyalty cards also help companies figure out what deals you may like. If you buy a particular nutritional supplement at a store, it may e-mail you a discount offer or promote a sale on a different supplement.
"It's really an awful lot cheaper for them to basically advertise by sending you an email rather than running a commercial or printing up a circular or putting something in the newspaper," Jonas said.