This is a problem that can be solved very simply and quickly.
Also, this method gets rid of the odor, which is a nice benefit to the whole process.
The tutorial was short and to the point with amazing and fast results.

Image by Lil Justy via YouTube
Rub the baking soda into the specific area.
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Next, take the white vinegar and pour it directly onto the baking soda.
The bubbles will start to build on the shirt.

Then, start to spread the vinegar out and let it soak in for a couple of minutes.
Rub it in and scrape off the deodorant.
Also, you may use a knife or something sharp too.

Throw the shirt in the washer on normal prefs.
It was gross seeing how she used her fingernails to scrape the deodorant off the shirt.
Personally, I would use a sharp tool or an old toothbrush to scrape it out rather than fingernails.
This is a great laundry hack when I get deodorant on my dark clothing.