Undecylenic acid is a fatty acid that works by preventing fungus from growing on the skin.

Fungi-Nail Pen (for the skin) is used to treat skin infections that are caused by fungus, such as athlete's foot, jock itch, or ringworm.

Fungi-Nail Pen may also be used for purposes not listed in this medication guide.

Follow all directions on your medicine label and package. Tell each of your healthcare providers about all your medical conditions, allergies, and all medicines you use.

Use exactly as directed on the label, or as prescribed by your doctor.

Do not take by mouth. Topical medicine is for use only on the skin.

Use only the Fungi-Nail Pen formulation made for the specific body area you are treating (such as your feet, groin, or nails). This medicine will not be effective if you do not use the correct formulation.

Do not allow a young child to use Fungi-Nail Pen without help from an adult.

Wash your hands before and after using this medicine.

If you are using Fungi-Nail Pen on your feet, apply the medicine to all areas including between your toes.

Do not cover treated skin with a bandage. You may cover the skin with a cotton gauze dressing to protect your clothing.

Avoid wearing tight-fitting, synthetic clothing (such as nylon) that doesn't allow air circulation. Wear loose clothing made of cotton and other natural fibers until the infection is healed.

Change your shoes and socks at least once daily while treating athlete's foot.

Call your doctor if your symptoms do not improve after 4 weeks of treatment.

Use this medicine for the full prescribed length of time, even if your symptoms quickly improve.

Store at room temperature away from moisture and heat. Do not freeze.

You should not use Fungi-Nail Pen if you are allergic to it.

Ask a doctor or pharmacist if Fungi-Nail Pen is safe to use if you have sensitive skin or allergies.

Do not give this medicine to a child without medical advice.

Ask a doctor before using this medicine if you are pregnant or breast-feeding.

Apply the medicine as soon as you can, but skip the missed dose if it is almost time for your next dose. Do not apply two doses at one time.

An overdose of Fungi-Nail Pen is not expected to be dangerous. Seek emergency medical attention or call the Poison Help line at 1-800-222-1222 if anyone has accidentally swallowed the medication.

Do not get Fungi-Nail Pen in your eyes. If contact does occur, rinse with water.

Get emergency medical help if you have signs of an allergic reaction: hives; difficult breathing; swelling of your face, lips, tongue, or throat.

Fungi-Nail Pen may cause serious side effects. Stop using Fungi-Nail Pen and call your doctor at once if you have:

• severe burning or stinging of treated skin; or

• any new skin symptoms.

Common side effects of Fungi-Nail Pen may include:

• irritation of treated skin.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.