Leaves of three — let them be! You've probably heard that little rhyme about poison ivy, the plant that can cause an itchy rash. But did you know that poison ivy, poison oak, and poison sumac all contain the same rash-causing substance?

It's called urushiol (say: yoo-roo-shee-ol), a colorless, odorless oil (called resin) contained in the leaves of the plants.

Look Out for Poison Plants

These plants can be anywhere — from the woods to your own backyard. The green leaves of poison plants blend right in with other plants and brush, so it's possible to sit down in a patch of poison ivy and not even notice. You might notice later, of course, when you start to itch!

And it's not enough just to know what one kind of poison ivy looks like. Poison ivy comes in several types — and may look different depending on the time of year.

The leaves of poison plants release urushiol when they're "injured," meaning if they get bumped, torn, or brushed up against. Once the urushiol has been released, it can easily get on a person's skin, where it often causes trouble. When the oil is released, the leaves may appear shiny or you may see black spots of resin on them.

It's also possible to get this kind of rash without ever stepping into the woods or directly touching one of the plants. Here's how: 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 your dog that likes to roam the woods! Urushiol even can travel through the air if someone burns some of the plants to clear brush.

An Allergic Reaction

Urushiol is considered an allergen because it causes an allergic reaction — the rash and sometimes swelling. Not everyone will get a reaction, but about 60% to 80% of people will.

This reaction can appear within hours of touching the plant or as late as 5 days later. Typically, the skin becomes red and swollen and blisters will appear. It's itchy, too. After a few days, the blisters may become crusty and start to flake off. It takes 1 to 2 weeks to heal.

Check With the 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. The doctor might want you to come in for an office visit.

If your rash was caused by poison ivy or a similar plant, the doctor may recommend cool showers and calamine lotion. In more severe cases, a liquid or pill medicine called an antihistamine might be needed to decrease itching and redness. A steroid (say: ster-oyd), another kind of medicine, may be prescribed in some cases. This medicine may be applied directly to the rash or taken in a pill or liquid form.

Preventing Rashes From Poison Plants

The best approach is to avoid getting the rash in the first place. Here are some good steps to take.

• Learn to identify poison ivy, oak, and sumac, so you can steer clear of them. (Be especially careful if the leaves look shiny.)
• Avoid areas where you know these plants live.
• Wear long sleeves and long pants when you're in areas that could contain poison plants.

If you come into contact with urushiol oil, try to wash it off your skin right away. But don't take a bath! If you do, the oil can get in the bath water and spread to other areas of your body. Take a shower instead, and be sure to use soap. And if your dog has been out exploring the woods, you might want to give your pet a shower, too!

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