What can cause a hangover?
Dehydration, hypoglycemia, acetaldehyde intoxication, dehydration, and vitamin deficiencies (such as vitamin B) are all theorized causes of hangover symptoms. One study shows approximately 25-30% of drinkers may be resistant to hangover symptoms. That leaves the majority of people prone to getting a hangover, about 70% of them.
Source: Wikipedia

History of hangovers
Hangovers of course have been around since people started drinking heavy amounts of alcohol. Originally, the term hangover was a 19th century expression describing unfinished business, something left over from a meeting or survival. In 1904, the meaning of a hangover to be the "morning after-effect of drinking too much first surfaced. When the ancient Assyrians felt the painful hangover from having to much merriment, they consumed a mixture of ground myrrh and birds' beaks. In the Middle Ages, European doctors recommended raw eel and bitter almonds for hangovers. Then Mongolians used to eat pickled sheep's eyes...ehhh. While China went with a dose of green tea which some still do today. Germans still eat Katerfruhstuck for their hangover, a postbinge breakfast that usually consists of herring, pickles and goulash. Russians on the other hand don't eat anything at all. They would jump in a sauna and sweat it all out, and sometimes whip themselves with birch branches to help aid blood flow. In 1845, Italian Bernardino Branca developed a hangover cure (cure-all) he called Fernet — an 80-proof concoction containing myrrh, cardamom, rhubarb, aloe, chamomile, peppermint oil and a number of other ingredients including grape-infused spirits and also some opiates. Bernardino used the drink to treat a number of ailments, including treating hangovers and treating cholera.
Source: Telegraph.co.uk
Source: Wikipedia
Source: Time

Use of Logos
Sources of the articles where Prickly Pear was mentioned, in regards to the Forbes, MSNBC, USA Today and U.S. News logos used on this page:
Forbes --- Ten Tried and True Hangover Cures
USA Today --- Hangover Helper: An extract of prickly pear cactus
MSNBC --- Prickly pear extract may be a hangover helper
U.S. News --- Hangover Cure: A prickly pear cactus could become a drinkers best friend

What is the definition of a hangover?
A hangover (also called delayed alcohol induced headache) is the explanation of the sum of unpleasant physiological effects caused by consumption of alcohol, usually in heavy amounts. Commonly reported characteristics of hangovers include sensitivity to light and noise, headache, lethargy, dysphoria, thirst and diarrhea dehydration, fatigue, body aches, vomiting, flatulence, weakness, hypersalivation, elevated body temperature, difficulty concentrating, sweating, anxiety, dysphoria, irritability, erratic motor functions (including tremor), trouble sleeping, halitosis, severe hunger, and lack of depth perception. Hangovers may also induce mental symptoms such as heightened feelings of anxiety and depression.
Source: Wikipedia

What are hangover remedies?
Hangover remedies are different things that help a hangover. A hangover remedy can range everything from drinking water to taking vitamin B, and there are lots of hangover pills, powders and drinks. Even "Hair of the Dog" is considered a hangover remedy, which is essentially drinking more alcohol to try and relieve symptoms of a hangover. Hangover remedies have been around for decades.
Source: Wikipedia