When I say that they are not interchangeable, what I mean is that although the drug is just the same, the 2 types of patches physically work differently. This means that they won't necessarily be delivering the same dose per hour.

The key feature of transdermal delivery (ie patches) is that they release a steady dose of medication over a set period. In theory, provided you always use the same delivery system, then you will get the same dose every hour and your body won't be subject to fluctuations in the level of the drug in your system. If you chop and change the delivery system (ie reservoir to matrix or vice versa) then you are at risk of receiving a different dose each hour. It may be minimal or your body may be insensitive to the difference so there are no noticeable issues. However, if the difference is significant or you are sensitive to even small fluctuations in the drug level, you may experience increased side effects or withdrawal symptoms. Both potentially dangerous and at the very least a bit unpleasant.

Transdermal delivery and the nature of how fentanyl is absorbed and used, means that its difficult to make rapid adjustments to fentanyl levels either up or down to manage any problems that do arise. For these reasons, it is advised that the different types of patch are not interchangeable.

It's possible to be swapped from one to another in a managed fashion but that would be a single swap over with the intention of sticking to the new delivery system for the longer term, not just changing each time the pharmacy has a different type of patch in stock. A managed swap would take into account any anticipated variations in the hourly dosage rate and compensate for that.

Hope that explains. I find the matrix patches are excellent - so unobtrusive and easy to tolerate since they are essentially no more than a little piece of Sellotape.