The repellent stick is a simple wooden stick with "mole rat repellent" (a combination of jet and psycho) on it. The stick is exceptionally effective against mole rats, as one hit causes an allergic reaction on mole rats, causing their bodies to smoke and their heads to explode after five seconds, while a second hit will cause the explosion to happen instantly. Do not be fooled by what Moira says when you first get this stick, as it's an experiment gone wrong.
If you use the stick on any other creature other than a mole rat, it will still cause the "smoking" effect, though it won't cause them to explode.
Upon completing the mole rat portion of the Wasteland Survival Guide quest, if the optional objective of using the repellent on seven more mole rats is fulfilled, the player is able to keep the Repellent Stick for future use. If the optional objective is not fulfilled, or if the quest is completed with a speech-based lie, Moira takes back the stick but will sell it to you from that point on, meaning you can still have it permanently if you accidentally (or intentionally) did the bare minimum. If in the future you don't have the Repellent Stick and you want it, it can be found by lock-picking the upstairs desk (hard difficulty). Be careful while doing this, since if you take it this way it may not be able to be taken out of your inventory. It will be considered a quest item.
The repellent effect will not register if you incidentally occur instant kills of the mole rat, such as through the Sneak Attacks afforded by the Chinese stealth armor or Stealth Boy. You will not get credit toward the completion of the quest unless the repellent has the chance to work on each mole rat before they expire. So, when using this weapon to complete the Wasteland Survival Guide quest, do not use sneak attacks to kill the mole rats.