Many generic products look a lot like their brand-name counterparts, probably to make you notice the less expensive version while shopping for the name brand. But generic prescription drugs don’t vie for attention on store shelves, so they have no reason to mimic their competition — and, in fact, they often look quite different.

Unfortunately, that happenstance of marketing has led to a risky situation for the two to three million Americans who take warfarin (Coumadin), a commonly used blood thinner. Generic warfarin — introduced last year — has a different shape and markings than brand-name Coumadin (see pictures), though they contain the same drug. So, some people who’ve switched from one to the other and had some of their old prescription left over have accidentally taken both, which can cause dangerous bleeding.

Left: Coumadin (brand-name warfarin)
Right: Generic warfarin

There isn’t much of a price difference between generic warfarin and Coumadin — about $5 a month — so it might not be worth it to switch to the generic. But if your drug plan compels you to switch (remember to notify your doctor), or you decide to take Coumadin instead of the generic, take some preventive measures:

Put your new prescription somewhere out of the way until you’ve used up your old prescription, and try marking both bottles with a big “X” or a piece of colored tape to remind yourself that they’re the same drug and should never be taken simultaneously.