Indication: This product is used to relieve itching and pain from certain skin conditions, such as scrapes, minor burns, insect bites, rash due to poison oak, poison ivy, or poison sumac.

Insight:

Menthol is known as a counterirritant. It works by causing the skin to feel cool and then warm.
These feelings on the skin distract the patient from feeling the ache or pain.
Camphor helps to relieve itching.
Adverse effects may include redness, warmth, and irritation.
Directions: Topical medicine is for use only on the skin. Do not use on open wounds or on sunburned, windburned, dry, or irritated skin. Rinse with water if this medicine gets in your eyes or mouth.

Sources:

Sarna Original Topical: Uses, Side Effects, Interactions, Pictures, Warnings & Dosing - WebMD
Sarna Uses, Side Effects & Warnings - Drugs.com
sarna lotion - Google Search