Learn how to avoid a painful case of poison ivy — or treat it correctly, if necessary.

Earlier this summer, I contracted a case of poison ivy that was so bad, I woke up in the middle of the night clawing my forearms with my fingernails — scratchin’ like a hound, as The Coasters famously sang. I thought I knew how to avoid this noxious plant, which I’ve spotted in my neighborhood and on nearby hiking trails.

But as it turns out, misconceptions about getting and treating poison ivy are common. Below, two Harvard dermatologists share the facts to help you avoid poison ivy — or manage it if you get it.

How can you be exposed to poison ivy?

“When I was growing up, my parents told me you could only get poison ivy if you touched the leaves when they were red and shiny,” says Dr. Abigail Waldman, a dermatologist at Harvard-affiliated Brigham and Women’s Hospital. That is not true, she emphasizes. Urushiol, the oily resin that triggers an allergic reaction, is found throughout the year in all parts of the plant: the leaves, stems, bark, fruit, and roots.

Brushing against any part of the plant or touching anything that’s been in contact with the plant — your clothes, shoes, garden tools, or your pet — can cause an allergic reaction. I didn’t recall having been near any poison ivy, but it’s possible my dog wandered near a patch and I got the oil on my hands and arms while removing her harness when we got home from a walk.

Wash up quickly to avoid a case of poison ivy

If you’re exposed to urushiol, wash the area with soap and water (dish soap is best) as soon as possible, ideally within an hour. “You also need to wash anything that might have been contaminated — not just your clothes but other things you don’t normally wash, like your jacket and shoes,” says Dr. Waldman. Once my rash erupted and I realized I had poison ivy, I scrubbed down my dog and washed her leash and harness, along with the running shoes I wear on walks.

A rash may not develop right away

You can develop symptoms — a red, itchy rash that sometimes features blisters — from four hours to four days after you’re exposed to urushiol. Why the long lag time? The rash caused by poison ivy is a form of allergic contact dermatitis. It’s mediated by T cells, the immune cells that recognize and attack foreign substances — in this case, skin proteins that react with urushiol.

“T cells take 24 to 96 hours to ramp up in the body, which is why this is also known as a delayed-type hypersensitivity reaction,” says Dr. Jeff Yu, assistant professor of dermatology at Harvard Medical School. Symptoms take longer to show up after your first exposure to poison ivy. “But once your body has been exposed and reacted several times, it tends to show up faster because your body ‘remembers’ the rashes and has T cells ready to go,” he explains.

This reaction differs from more familiar allergies, such as those to cats, dogs, or mold, which are immediate hypersensitivity reactions. These occur within minutes and are mediated by histamine, a chemical that plays a role in reactions such as a runny nose and watery eyes.

Because I didn’t realize I’d been exposed to urushiol, I had unwittingly transferred the oil to other parts of my body, including my torso. Fortunately, once the oil is washed away, the rash is not contagious — you can’t spread it to other parts of your body or to other people, says Dr. Waldman.

Treating poison ivy: Start with over-the-counter remedies

For milder cases of poison ivy, simple remedies may suffice, says Dr. Yu, who treats patients at the Contact Dermatitis and Occupational Dermatology Clinic at Massachusetts General Hospital. A thin layer of topical 1% hydrocortisone cream on the affected area may help, if you have a very mild case limited to one area on an arm or leg, says Dr. Yu. You also can try topical anti-itch products, such as calamine lotion or Sarna lotion with menthol, he suggests.

For me, hydrocortisone cream didn’t help. “Once poison ivy spreads, topical hydrocortisone is like sprinkling water on a wildfire,” says Dr. Yu. A friend dropped off a rash-relief spray that provided soothing but short-lived relief. Another suggested I try taking the antihistamine diphenhydramine (Benadryl), an over-the-counter allergy drug. That’s not helpful because histamine doesn’t have a prominent role in poison ivy, says Dr. Yu. “The only benefit you might get from Benadryl is feeling sleepy,” he says. And in fact, it did make me sleepy, but I was still itchy and very uncomfortable.

So, what works if over-the-counter treatments don’t help and symptoms are interfering with your life? A health care provider may prescribe oral steroids. Indeed, soon after I visited an urgent care clinic and began taking the oral steroids a doctor prescribed for me, I felt much better. Be sure to follow the dosing instructions carefully: Oral steroids must be tapered, which means gradually taking lower doses over a period of three weeks to avoid a possible flare-up. And always consult a doctor if a poison ivy rash involves sensitive parts of the body like the face or groin.

About the Author

Julie Corliss, Executive Editor, Harvard Heart Letter

Julie Corliss is the executive editor of the Harvard Heart Letter. Before working at Harvard, she was a medical writer and editor at HealthNews, a consumer newsletter affiliated with The New England Journal of Medicine. She … See Full Bio View all posts by Julie Corliss