There are two factors that affect patch performance that involve location.
The first is the purely mechanical. It doesn't matter how much the patch is capable of delivering if you can't keep it flat and adhered fully. This means that folds, rumples, areas of high clothing friction, and places with thick fine body hair are just not going to work well no matter where they are. Upper arm and thigh are some off-label locations that some women have reported satisfactory results with when they don't have enough suitable real estate in the mid-body areas.
As a rule, though, best results are obtained in the outer half of each body half, but not so far "around the edge" that there's a lot of curve to the location and not directly over an area where there is little padding over the bone or over large muscles.
The other location factor is that studies by manufacturers have shown that patch delivery (same patch, same dose, same degree of adhesion, different location) varies by 17-25% depending on whether it's on the front of your body or back. That is, locations on your butt deliver more to your system, from the same patch, than if it were placed on the front of your body.
Our speculation on this—and that's all that it is because it's not elucidated by the manufacturers—is that the large muscles in the butt (the same ones that make it a good location for shots) cause more circulation to the area when you move around, leading to the hormone being pumped away from the patch location at a more rapid pace. On the belly, unless we're seriously into muscle-building, that's less likely to be the case. This would correspond to the experiences women with little body fat or women who undergo a lot of weight/fat loss see, where they find that their patch dynamics are changed such that they may not be able to use patches satisfactorily any longer.
Now it's true that not every manufacturer includes this dosing variability in their product information, and that some state that the patch can only be used in one particular location. These cautions have more to do with the testing and approval process than actual patch dynamics, however. If the manufacturer does not go to the expense and time of specifically testing their product in multiple locations, they can't claim those results—only the ones they actually demonstrated in their FDA application can show up in their licensing. And because they can only claim those specific results, it's obvious that they can tell you that only that location works as specified. It's not that other variations don't work, won't work or have been determined not to work: it's just that they don't have the legal right to allow even that they might work without demonstrating this through specific testing.
There is nothing in the physiology of hormone uptake or the mechanical delivery of hormones from patches that would make one work in a specific location and not the other. In practical terms, it is entirely reasonable to generalize from the specific test data to general patch performance, leaving aside delivery variations due to specific patch qualities.
Further, this can be expanded to general considerations in patch placement. Aside from on or within the immediate circulatory feeders of breast tissue, there's virtually nowhere that is an absolute contraindication to patch placement. We'd have second thoughts about over some moles (for the same reasons as avoiding breast tissue), but otherwise, if the bottom of your foot is the only available real estate and you want to use a patch, there's not an obvious reason beyond the mechanical why that won't work. We have no idea what the delivery efficiency would be, but that can be deduced in practice.
The point is, then, that different underlying tissue types seem to affect your uptake (and provide a subtle way of manipulating that intake) for any two locations where the mechanics of sticking are equivalent. But because delivery is first and foremost dependent upon patch adhesion, locations where a patch won't stick just aren't in the running.
For more information on alternate locations and their dose delivery, plus more documentation on the importance of the tissue type underlying a patch, check out this other post looking at what we can learn from contraceptive patches. For help doing the math on how much difference location makes to a specific patch dose, please join us on our discussion forums.
