It would probably be a very good idea to refrain from using topical nitroglycerin cream as a sexual lubricant.
While odds are good that an otherwise healthy individual would escape any serious complications from doing this, it is best not to take chances. Nitro is rapidly absorbed into the bloodstream in whatever form it is prescribed. This fast response is essential to the treatment of the specific symptoms -- usually angina.
The thing is, anal absorption of medications can be risky. A measured, prescribed dose of nitro for a person who needs it is not without its own accepted and expected risks and side-effects. But squeezing out a glob of this stuff and rubbing it into your sphincter by repeated sexual thrusting -- and probably using much greater amounts that would be prescribed for an actual patient... well... it doesn't take a genius to figure out the inherent dangers of such a thing.
Mixed with Viagra or poppers or worse -- both... this could lead to potentially very nasty results.
As for other, non-sexual lubricants -- yes, they can disrupt the normal intestinal bacteria flora, but in most situations this isn't going to be anything so serious as to cause major problems. Lots of fisters use Crisco with no ill effects. It would, I presume, be a bitch to clean up. Yet barring any pre-existing conditions or propensity toward infection in an individual with a healthy immune system -- it's probably nothing that requires a dire warning.
That being said, for all intents and purposes, I see no reason why we cannot stick to sexual lubes which are made specifically for sexual activity. Other stuff is fun and MOST of it is harmless, albeit sometimes sloppy. But there are tons of different lubes out there. Elbow Grease is a pretty good alternative to Crisco for most folks, though serious fisters have mentioned that they would never use anything BUT Crisco. Water soluble lubes are, of course, easiest to clean, though they have their own drawbacks such as faster drying time.
Just please purchase TWO cans of Crisco if you are going to use it -- one for the bedroom which will STAY in the bedroom, and one for the kitchen which will STAY in the kitchen. Pubic hairs found in your pie crust can be a serious breach of etiquette at a dinner party...
And I'm not the resident doc nor even a "medical type." But the general info here is sound, I believe.
Here's a link to some basic, non-sexual info about topical nitro:
http://www.personalmd.com/drgdb/9048.htm
Anus Maximus -- wasn't he a gladiator? I think I saw a Discovery Channel show about him. Or maybe I'm mistaken...