Too many people believe in Jewel weed to not take it seriously, and scientists have taken it seriously, but so far have observed no benefit. That does not mean there is not a benefit in the field, but it may be due to merely applying a liquid to the area and rubbing which may remove some urusiol.
There are two controlled studies that I know of and in both, Jewel weed did not perform any better than a saline control or no treatment.
LINK: The effect of jewel weed in preventing poison ivy dermatitis. We designed a double blind, randomized and controlled clinical trial in human volunteers to test the effect of jewel weed juice in preventing poison ivy dermatitis. Twenty-five subjects were enrolled and had three areas on their forearms exposed to poison ivy leaves. The exposed areas received one of three applications: jewel weed juice, saline, or no agent. The subjects were assessed at 48, 72 and 96 h for severity of dermatitis and symptoms.
No significant differences were found between the three categories in severity of dermatitis or degree of symptoms. With this model, it appears that jewel weed is not effective in preventing poison ivy dermatitis.
LINK: Treatment of Poison Ivy/Oak Allergic Contact Dermatitis With an Extract of Jewelweed. A randomized, double-blinded, paired comparison investigation was performed. Ten adult volunteers were patch tested to urushiol, the allergenic resin in poison ivy/oak. For each volunteer, one patch test site was treated with an extract prepared from the fresh stems of jewelweed; the remaining site was treated with distilled water to serve as a control. Sites were examined on days 2, 3, 7, and 9 with reactions graded on a numerical scale.
Results: All subjects developed dermatitis at each patch test site.
There was no statistically significant difference in the objective scores at the sites treated with jewelweed extract versus the distilled water control sites.
I am not trying to dis the experience of those who feel that jewel weed has worked for them, only to let folks know that there is no scientific support for it so far and to caution you to not depend on jewel weed to prevent or treat a reaction.
In my view, it is possible to get some benefit from jewel weed because it is a very juicy plant and could be used to wash off the urushiol. What I recommend is to use any liquid, and if jewel weed is available go for it, but rub the liquid vigorously and then wipe with something. Do this at least three times and you may physically remove the urushiol. But it is unlikely that jewel weed has any kind of neutralizing or anti-inflammatory affect on urushiol.