worry vs niggle

worry is a noun but niggle is not a noun.

Word	Noun	Adjective	Verb	Adverb
worry	Yes	No	Yes	No
niggle	No	No	Yes	No

As verbs, niggle is a hyponym of worry; that is, niggle is a word with a more specific, narrower meaning than worry:

Other hyponyms of worry include fret, fear, fret, fuss, obsess.

worry (noun)	niggle (noun)
something or someone that causes anxiety; a source of unhappiness
a strong feeling of anxiety

worry (verb)	niggle (verb)
lacerate by biting	argue over petty things
touch or rub constantly	worry unnecessarily or excessively
be worried, concerned, anxious, troubled, or uneasy
disturb the peace of mind of; afflict with mental agitation or distress
be concerned with
be on the mind of