Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency help for difficulty breathing, rapidly increasing mouth or throat swelling, or an inability to swallow saliva.
Seek prompt medical or dental advice if you develop significant pain, swelling, fever, pus, repeated bleeding, difficulty drinking, or signs of dehydration.
An ulcer, lump, or red or white patch that does not heal also needs assessment.
These are not typical features of hairy tongue and may signal another urgent problem.
Arrange a routine appointment with a dentist or primary care professional if the change is new and you are unsure what it is, or if it does not improve after a week or two of gentle tongue cleaning.
Routine assessment is also appropriate if it keeps returning, causes troublesome gagging, affects eating, or is accompanied by ongoing dry mouth.
Every sign above is quoted from the sofidex article on Hairy tongue. If you are not sure, err upward — it is always reasonable to be checked.