Thanks for your query.
Itchy rash on the chin can be due to many diseases as you have rightly mentioned- the allergy to anything, poison ivy and herpes.
Herpes rash is very typical.
I think it is better to apply nothing as most of the rashes are self-limiting.
Please consult a Doctor as a visual impression of the lesion alone can get us the diagnosis.
You can also post a picture attached to such a query so that a clear picture can give us an idea as to what it is.
I hope this answer helps you.
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