There is not one unique phrase in use for this concept. I would say that "methemoglobin forming agents" is the most common one (sometimes with "compounds" instead of "agents"). There is also the more cumbersome phrase "methemoglobin forming cyanide antidotes".

"Methemoglobin formers" is also in use, and of course Lisa's and Sven's suggestions are also correct.

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Note added at 1 hr 18 mins (2004-09-24 08:39:07 GMT)
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Forgot that you might want to know what methaemoglobin actually is. (When I taught at med. school I ran a hemoglobin lab and short lecture 6 times every semester, so I happen to know it quite well.)

Cyanide is poisonous because it binds very strongly to heme groups and them from doing there jobs. This is true about the heme groups in hemoglobin (which are supposed to bind to oxygen to transport it to all tissues), but even more important is that it happens in the cytochromes, which are important to the cellular respiration (they ar located in the mitochondria - the \"power plants\" of the cells. When they can\'t do their job, the cells \"suffocate\" (or get power depleted).

Well, the way to treat this poisoning is to have another substance bind the cyanide even stronger. And this is what methemoglobin can do. Methemoglobin is really a kind of \"damaged\" hemoglobin. It is a hemoglobin molecule that has been oxidized. This means that it has lost its oxygen carrying capability and thus is useless. But for one thing: it binds immensely strong to cyanide. Thus, as the methemoglobin binds up all the cyanide ions, the cytochromes and the ordinary hemoglobin can work properly.

A methemoglobin-forming agent, does not contain any methemoglobin per se. Rather it is an oxidizing agent that oxideizes a part of the body\'s own hemoglobin, to form methemoglobin.

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Note added at 1 hr 22 mins (2004-09-24 08:42:48 GMT)
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Oops, lots of typos and sloppy language. One particular sentence above should really read \"Cyanide is poisonous because it binds very strongly to heme groups and prevents them from doing their jobs.\"