Yellow toenails are usually a sign of trouble to come. Although harmless bacterial colonization can cause a color change in nails (usually green), and there is a rare condition called yellow nail syndrome, by far the most common cause of yellowish or brownish discoloration in the toenails is fungus infection. Many fungi produce colored pigments when they are growing well - Penicillium, for instance, the fungus that produces penicillin, commonly produces a deep green pigment. The fungi that like to grow in nails commonly produce yellowish or brownish colors.

To determine whether, in your case, a fungal infection is the reason for yellow toenails, you should visit your doctor and get a professional diagnosis. Since fungal nail infections are relatively common, your doctor will have seen yellow toenails before, and will be able to judge whether your nails are infected with fungus or whether you have some other problem that requires different treatment. If there is still doubt, a sample of your yellowish toenail can be taken and sent to the laboratory for testing.

Yellow toenails are not necessarily uniformly colored. Only some toenails may be affected - sometimes only one - and the fungus may only be growing in part of the nail, producing a yellow toenail streak. Untreated, infections that involve only part of the nail will usually spread to cover the whole nail, and generally advance towards the nail root over time. Once you know for sure that fungus is the reason for yellow toenails in your case, don't delay treatment. Infections are more easily treated if they do not involve the lunula (the whitish crescent shaped area near the cuticle).

You will have to decide between various types of treatment for yellow toenails. Your doctor can prescribe an oral medication that is taken daily for a few months to a year to get rid of the infection. Many people avoid the oral prescription drugs because they are expensive, and because of the potential for unpleasant side effects. There is also a topical prescription nail lacquer that is painted on, but this can only be used for yellow toenails, or a yellow toenail streak that does not involve the lunula (the whitish crescent-shaped region near the cuticle).

Besides the prescription drugs, natural remedies probably work better than over the counter antifungals for yellow toenails and any yellow toenail streak. This is because topical ointments and lotions don't get through the nail to kill the fungus. Natural and home remedies attempt to soften the nail for better penetration, and often contain natural ingredients that have been found to be effective against fungus. Some are even backed up by scientific study.