Walgreens, Rite Aid, and CVS all had programs that can allow you to get items cheap or free after a rebate, or coupon good for your next purchase.
CVS uses a membership card, like a grocery store's loyalty card, that you can scan at the kiosk near the door in most stores, and that will print out Extra Care Bucks (ECBs) on some items. ECBs can be used like cash on any item in the store, and which expire after a month. For example, maybe this week the deal is on Colgate Total, which costs $3 per tube, and gives you $3 back in ECBs. There's a limit of two per card. You can spent $6, buy two tubes, and have $6 in ECBs to use on whatever next week's deal is. Or, you can spend $3, get $3 back in ECBs, and then spend those ECBs on another tube, and have $3 to spend next week. If you can find a coupon (printable, or from an insert) for that toothpaste, you could, say, spend $2 after coupon, and get $3 back. If you get the Green Bag Tag (it's a leaf shaped card, I put mine on my key chain), and scan it every time you use a reusable bag, you'll get $1 in ECBs for every four visits.
Walgreens uses Register Rewards, which are similar, but the rules are more restrictive. You don't need to use a membership card, but the RRs expire after two weeks, and are treated as manufacturer's coupons. So, if you're making a purchase, and want to use a RR and a manufacturer's coupon, you need to have at least two items. If you buy an item that generates a RR, and use it to purchase a second item, you won't get a second RR.
Rite Aid uses . . . damned if I know. They don't have any around here.
The idea of the Drugstore game is to use your ECBs or RRs on items that generate more ECBs or RRs, rather than on general merchandise. So, buy your toothpaste this week, use the ECB or RR on some shaving cream next week, maybe there's nothing worth buying the next week, so you skip it, but then the next week, you buy some obscure brand of tampons because the thing's about to expire, and you don't want to waste it. Just keep going on that way, choosing the free or super-cheap item of the week. You can donate anything you don't like. Blogs like iheartcvs.com and iheartwags.com will give you lists of current and upcoming deals. Pretty easy, and you'll end up with a lot of toothpaste, even if you only frequent one store.