Many thanks for these thoughts. I'm in the UK, so my responses probably need to be read in that light.
1) The UK version of the cream is marked "For use only as directed. For external use only*." No further instructions are given.** My own GP gave me no specific instructions either, except to say that the cream is meant to relax the internal anal sphincter.
This article in the WSJ pinpoints some of the problems and dangers for patients when the instructions for medications are unclear, ambiguous or non-existent.
2) That's a great suggestion. I wonder how flexible the UK's health service can be in a case like that. The UK version of the cream contains, in addition to diltiazem hydrochloride: "cetomacrogol, emulsifying wax, white soft paraffin, liquid paraffin EP, phenoxyethanol and purified water." I'm not sure which of the latter ingredients forms the base. Maybe the emulsifying wax could be the irritant?
3) Having tried glyceryl trinitrate (GTN .2%) with few results except some headaches, I switched to diltiazem, with the rather severe side effects mentioned earlier. I believe that like diltiazem, in the UK nifedipine is licensed only for the treatment of angina and hypertension. Also, I haven't heard of it being prescribed for AF here, but could ask my GP.
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*Does it mean "use only on the skin"?**Interestingly, an information leaflet (PDF) published by a London hospital gives the following instructions: Apply the cream just inside and around the anus (back passage) as instructed by your doctor. A finger covering such as cling film, disposable glove or finger cot may be used to apply the cream (you can get finger cots from a local pharmacy or surgical supplies retailer).