Strictly speaking, our rule is that, regardless of proprietary status, we don't allow a word normally written with a capital letter. In practice though, the rule applied is more like we don't allow a word that is
almost always written with a capital letter. So we accept quite a few words that are sometimes written in all lower case letters, even if they are more often written with an initial capital. I'm especially likely to accept a word if at least one standard dictionary lists it without a capital.
It could be argued that a proprietary name is not a word at all - that it is a name. But I don't think such an argument applies to
taser, since it is normally used as if it were a word: it can be either a noun or a verb, and in either case is frequently inflected in accordance with normal English rules. If the desires of TASER International were respected by everyone, the issue wouldn't arise: the word would be written all in capitals and their products would be known as "TASER Conducted Electrical Weapons" or "TASER CEWs" for short. Hence we would never see
tasers,
tasered or
tasering.
Interestingly, the
US English version of the online Oxford Dictionary has a capital
T for the noun, but the verb is in lower case. The "British and World English" entry has both verb and noun in lower case, although the noun is still labeled as a trademark.
I did consider whether
taser should be common,
several years ago. My conclusion was:
So I think it is probably time taser was treated as a common word. However, the verb forms, tasered, tased, etc, are used much less frequently and should probably remain rare.
However I didn't actually change anything. (Probably the reason I overlooked it was that the issue was raised well into a thread on another topic.) I still think
taser should become common, but I'm not so sure about leaving
tasered and
tasering as rare: I feel maybe they should be common too. Any thoughts?