As Dr Paul has explained, the appearance of a brown line on your nail, known as melanonchia, is due to melanin within the nail. It's most often due to non-worrying causes. These include:
- Moles
- Freckles
- Fungal infection
- Pregnancy
- Trauma/injury (eg. from nail-biting)
- Bleeding under the nail
Melanonchia due to moles commonly affect the thumbs, index fingers and big toes, like in your case.
Of course, as you may have read, the internet will say that the big C word is a possible cause too (melanoma, a particularly aggressive type of skin cancer).
Fortunately, melanoma is very uncommon in Asia, including Singapore.
The best way to know for sure would be to have it checked out by a friendly dermatologist, who will use a dermascope (kinda like a special magnifying glass to look at skin/nails) to ascertain the cause.
If the brown line is due to a fungal infection, then yes, topical applications will help to get rid of it. This will take a long while however, as unlike your hair, nails grow really slowly.
In the mean time, you may wish to check out this post on moles, cancerous moles, and mole removal in Singapore to find out what doctors do when we come across a suspicious mole.
Cheers
