Leaves of three, let them be! You've probably heard that little rhyme about poison ivy. But did you know that poison ivy, poison oak, and poison sumac all contain the same rash-causing substance? It's called urushiol (pronounced: yoo-roo-shee-ol), a colorless, odorless oil (called resin) contained in the leaves of the plants.

What Are the Signs and Symptoms of Poison Ivy?

Urushiol is considered an allergen because it causes an allergic reaction — which takes the form of a rash, itching, and sometimes swelling. Not everyone gets a reaction to urushiol, but about 60% to 80% of people do. This reaction can appear within hours of coming into contact with urushiol or as late as 5 days later. Typically, the skin becomes red, itchy, and swollen and blisters will appear. After a few days, the blisters may become crusty and start to flake off. The rash that people get from poison ivy takes 1 to 2 weeks to heal.

Should I See a Doctor?

It's a good idea to consult with your doctor if you have any kind of rash, especially if you have a fever too. If your doctor determines that a rash has been caused by poison ivy or a similar plant, he or she may tell you to take cool showers and to use a soothing lotion, such as calamine lotion. In more severe cases, doctors may prescribe pills or creams that contain antihistamines or steroids (not the same type of steroids that bodybuilders use!) to decrease itching and redness.

Can I Prevent It?

Poison ivy can grow anywhere — from the woods to your own backyard. And it's hard to identify: Not only can the green leaves of poison plants blend right in with other plants and brush, but there are several types of poison ivy, and each one can look different depending on the time of year.

It's also possible to get a rash from poison ivy without ever venturing into the woods or directly touching a plant. Urushiol can be transferred from one person to another. Plus, a person can pick it up from anything that's come in contact with the oil, including pets. Urushiol can even travel through the air if someone burns some of the plants to clear brush.

The leaves of poison ivy plants release urushiol when they're bumped, torn, or brushed against. (When the resin is released, the leaves may appear shiny or you may see black spots of resin on them.) Once the urushiol has been released, it can easily get on a person's skin. Here are some tips to help you avoid getting a rash from poison ivy:

• Learn to identify poison ivy, oak, and sumac, so you can steer clear of them. (Be especially careful of plants if the leaves look shiny.)
• Avoid areas where you know there's poison ivy.
• Wear long sleeves and long pants when you're in areas where poison ivy might grow.
• If your dog has been out exploring the woods, give him a shower to wash off any urushiol oil that may be on his coat.

If you come into contact with urushiol oil, try to wash it off your skin right away by taking a shower and using lots of soap.

Reviewed by: Patrice Hyde, MD
Date reviewed: October 2010