Citrus limon extract reduces pain, but does not make you drowsy
Athletes who want to push their pain threshold have a love-hate relationship with painkillers. The drugs undeniably work, but they also make you drowsy and slow down reaction time. By coincidence we came across an older Brazilian animal study that suggests that an extract from Citrus limon reduces pain stimuli, but does not make you drowsy or sluggish.

The researchers gave mice nothing, or orally administered them different doses of Citrus limon oil. The researchers used oil made from the leaves of the Brazilian Citrus limon tree. It consisted of 53 percent limonene, 10 percent geranyl acetate and 7 percent translimonene oxide.More detailed information about the oil can be found in the original publication. It's on the web.
If the mice had been adult humans, they would have gotten about 400-1200 milligrams of Citrus Limon oil.

One hour after the administration of Citrus limon, the researchers injected the lab mice with an acetic acid solution. This causes severe pain. Animals then literally squirm in pain. Measuring these body movements is a a barbaric method of measuring pain, which scientists are fortunately using less and less often.
Citrus limon oil reduced the number of times the mice writhed in pain [Writhings]. The oil worked about as well as morphine. When the researchers gave the test animals not only Citrus limon oil, but also naloxone, the pain-relieving effect of Citrus limon oil largely disappeared.

Naloxone blocks the endorphins receptors. Endorphins are pain-relieving molecules that the body produces itself. Pain killers such as morphine work via the same receptors. Citrus limon oil apparently contains substances that interact with the endorphin receptors.
The researchers got their mice to balance on a rotarod after administration of Citrus limon oil. This is a machine with a revolving rod. The machine registers how long it takes for a mouse to tip off the rod.
Pharmacologists use a rotarod when they want to know whether an agent will be making its users drowsy, or slowing down their reaction speed. However, Citrus limon oil had no effect on the time the mice could balance on the rotating rod. Citrus limon oil apparently reduces sensitivity to pain, but has no adverse effect on reaction speed.

J Biomed Biotechnol. 2011;2011:678673.