Poison ivy
You can look but you’d better not touch
by John Kraft, MD and Charles Lynde, MD
Vol.17, No.05, May 2009

Poison ivy dermatitis is a classic example of an allergic contact dermatitis, or a delayed-type hypersensitivity to an allergen that comes in contact with the skin. On the first exposure, a patient may become sensitized but not develop a visible reaction. On subsequent exposures in a sensitized patient, an eruption will occur quite readily.

Poison ivy, Toxicodendron radicans, is a plant in the same family as poison oak and poison sumac. It produces a resinous sap containing urushiol, a group of catechols that trigger the allergic contact dermatitis. Other plants also produce catechols similar to urushiol in poison ivy (see Table 1). A person is much more likely, however, to be sensitized to poison ivy, oak or sumac than any of the cross-reactors.

Since the urushiol in poison ivy is mainly found within the sap of the plant, touching injured plants can result in greater exposure to the allergen. Higher humidity and temperature also influence the quantity of allergen available. Direct contact exposure can occur after touching pets or clothing that rubbed the plant. Aside from direct contact, other routes of exposure include smoke and airborne inhalation. Burning poison ivy can produce severe lung injuries.

Clinical features

After exposure to urushiol, an acute dermatitis appears at the sites of contact within 2 days and peaks within 2 weeks. Without treatment, lesions ordinarily last for 3 weeks.

Itching is usually the first sign, and can be severe. Erythema, dermal edema, vesicles and bullae soon follow.

Scratching frequently transfers the allergen to different sites, and can make the diagnosis more difficult. The classic presentation of linear vesicles or bullae is due to transfer from scratching.


Diagnosis is generally clinical, based on history of relevant exposures and skin examination. A skin biopsy is seldom necessary to make the diagnosis, but when done, can show acute, extensive spongiosis with intraepidermal vesicles or bullae.

Although patch testing often helps diagnose allergens in allergic contact dermatitis, it’s not helpful for suspected poison ivy dermatitis. It can’t distinguish between poison ivy, oak or sumac. There’s also a high risk of sensitization.


Ideally, the best strategy is avoiding the plant. It’s easily identified as a woody vine with leaves in groups of three; hence the expression: “leaves of three, let it be; leaves of four, eat some more.” This expression doesn’t hold for poison sumac, which has up to 13 leaflets on a single branch.

Barrier creams exist that patients can apply before going into the woods. They may prevent absorption of the allergen into the skin. An example is Ivy Block, an organo-clay preparation (5% quaternium-16 bentonite), that should be applied 15 minutes prior to exposure and repeated every 4 hours. Gloves can also be helpful, but ensure that they’re vinyl as urushiol can pass through rubber.

If a patient comes into contact with poison ivy, and presents with the classic acute pruritic eruption of erythematous linear vesicles and bullae, good treatment options are available. If untreated, the lesions and itch can persist for up to one month, and scratching frequently transfers urushiol to other areas on the body. With treatment, lesions and itch rarely last beyond 2 weeks.

Urushiol is rapidly absorbed into the skin. Attempts to wash it off are frequently futile. But early intervention can be beneficial. Aggressive washing with plain water followed by a soap and water wash removes half of the allergen load 10 minutes after exposure, 10% at 30 minutes, and has no benefit after an hour. Patients should wash all clothing, fabrics such as bed sheets, and even pets with which they came into contact after exposure.

Patients can use cool wet compresses, lotions, or wet to dry dressings for local relief. Antihistamines may be used as needed for itch. Super-potent topical steroids can be used on acute blistering areas and may be sufficient if the reaction is localized.

Most people will require a tapering course of systemic corticosteroids starting with 0.75-1 mg/kg/day. They should be warned about potential side effects of corticosteroids, especially ones that are serious, though rare, such as avascular necrosis of the hip. Treating for 5 days is often insufficient as new lesions could develop after this period. One option may be prednisone 0.75-1 mg/kg/day decreasing by 10 mg every 2 days until zero.

Patients should avoid other plants in the Anacardiaceae family that can cross react with poison ivy such as mango trees, cashew trees, Japanese lacquer, and Ginkgo biloba.

Not only is poison ivy allergic contact dermatitis very uncomfortable for the individual, but there are a number of potential complications that can occur if it’s not treated appropriately. These include secondary bacterial infections, erythema multiforme and urticaria.

Charles Lynde, MD, FRCP(C) is an assistant professor of dermatology at the University of Toronto.

John Kraft, MD, is in his third year of the Dermatology Residency Program at the University of Toronto.

column image
Poison ivy dermatitis on torso
Table 1: Plants that contain urushiol and its cross-reactors
  • poison Ivy
    • climbing
    • non-climbing
  • poison oak
    • Eastern
    • Western
  • poison sumac
  • cross-reactors
    • Japanese lacquer tree
    • Brazilian pepper tree
    • Hawaiian kahili tree
    • Indian marking tree
    • gingko biloba tree
    • mango
      • except the fruit and seeds
    • cashew shell oil
      • except the nut
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