Technically, not when one is starting a message of one word... if we were treating each response as a stand alone sentence--"Toxic."--then that would be correct (and we'd also need periods), but as it is, we're using words in a way that's rather grammar-free in that they aren't construed as being in a sentence or a paragraph. (Actually, this grammar free form of interrelation of words is somewhat what the idea of word association is based on: getting at the context apart from the language.)
toxicity