Since all patches currently on the market in the US and elsewhere all contain the same bioidentical estradiol, it would seem obvious that they would be, essentially, interchangeable. Aside from matters of adhesion, they ought to be so equivalent in function that if one fails to work out well for a woman, she might as well give up on the idea. Right?
No. In fact, the adhesive, which is the distinct and patented part of the patch (because the estradiol itself can't be patented), can have a significant impact on the delivery dynamic itself. For that reason, some women may change brands and find that while they had this or that symptom on one patch, their experience is completely different on another brand. In fact, there are two basic adhesive formulations, and these are what drive the differences. Because they are the major sellers of the two types, we'll look at the differences between Climara and Vivelle Dot.
Both of these are "matrix" patches (the hormone is spread through the adhesive and the adhesive is the delivery medium) as opposed to "reservoir" patches (the hormone is in a puddle confined and dispensed by a semipermeable membrane that lies between skin and puddle).
The difference between these two matrix patches, however, lies in the specific adhesive used and the way the hormone is mixed into it. With Climara (or the regular Vivelle or Esclim), the hormone is evenly mixed into (and dilutes, to some extent) an acrylic adhesive. This dilution effect (more hormone = less sticky) is why the Climara has to have a larger area per hormone quantity delivered. The manufacturer has hit on what seems to be an acceptable middle ground between these two demands, but it may not work out that way for any given woman.
With the Vivelle Dot, however, there are actually two adhesive agents. One is that same acrylic adhesive with the hormone mixed into it. The other is a silicon adhesive that is undiluted and thus can devote its entire strength to sticking. The two adhesives actually chemically repel each other rather than wanting to combine, which results in micro-puddles of the hormone-containing adhesive distributed throughout the (stickier) non-hormonal adhesive. This provides for a better adhesion although it may affect the allergic properties to do with the specific adhesives.
This is also cited as justification for the assertion that cutting the patch alters its dose delivery, on the premise that many of the micropuddles are cut and "open" along the new edge. We can't imagine that the scale of this could be of any functional implications, however, and the suggestion that one powder the cut edges to prevent lifting of the patch due to adhesive exposure along the sides would go a long way towards reducing this again.
There's not necessarily any predictive value in knowing this. Where it is useful is when a woman is having difficulties getting the coverage she wants from a particular patch but wants to continue with the patch form rather than giving it up to try another type of HRT. In these cases, then, switching from one sort of matrix patch to the other may provide a different enough performance. By the same token, women who change brands won't necessarily find them fully interchangeable and this is why.
