New Caprican loco weed
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During the initial colonization of New Caprica, indigenous fauna and flora were catalogued as quickly as possible for potential uses.
Even as crops were being planted in the alluvial deposits (with some of the produce immediately earmarked for the stills run by Saul "The Rummy" Tigh), the workers clearing the resilient native plant growth made a startling discovery. As the large piles of cleared weeds and roughage was lit on fire and began to burn, workers standing downwind began to feel ... funny. Valuable time was lost as an entire shift of workers ransacked supplies for any tasty edibles, stumbling about in a daze, laughing incessantly, and expressing their love repeatedly to anyone nearby (and sometimes, inanimate objects).
Exactly how the crop came to be common on New Caprica (and indeed, whether it is native to the planet) is unknown, however, several unofficial sources have suggested that the plant may have been brought to planets settled by the earliest migrants from Kobol as part of a terraforming process.
One person in particular was very interested in the properties of this particular plant: ex-President and former teacher Laura Roslin. As everyone knows, teachers really know how to party ("work hard, play harder" being a common motto), and Roslin is no exception. Her exploits while tripping the light fantastic on chamalla have been well-documented, as is her catchphrase, "Better Education (or Government) Through Chemistry," and have helped set the human race on the path to Earth (or to extinction, depending on what propaganda mill you shop at).
Roslin immediately obtained seeds and enough raw leaf to last any three heads a lifetime and had it transported, under the cover of mid-afternoon, to her secret grow site, later referred to by Roslin as "my cabin in the woods by a stream."
Her plans were foiled by the untimely arrival of the Cylons and the subjugation of the remaining humans - President Gaius Baltar being too busy drinking, popping pills, smoking (likely fumarella leaf or maybe even loco weed), and frakking his way into a stupor to do any real governing (TRS: "Lay Down Your Burdens, Part II").
The Cylon Occupation
The Cylons, naturally, outlawed the weed, and basically anything else that allowed the humans to have any fun or see any relief from their dreary existence. Alcohol, however, remained legal. Several demonstrations were held, requesting the use of loco weed for medical purposes at the very least. The Cylons response was in character: brutality. It is thought that the list of prisoners to be executed contained many weed smokers and legalization activists, as well as the typical political prisoners. Tom Zarek was probably a double-threat (TRS: "Precipice").
Return to the Fleet
It is unknown whether seeds or plant stock of New Caprican loco weed were brought from the planet when the humans returned to the Fleet. It's likely, given her history, that Roslin has them growing as house plants on Colonial One. With the recent return of her cancer, Roslin is apt to claim the use of her stash is for "medical reasons". 
A critical food shortage (TRS: "The Passage") may have been caused by overeating due to what has been termed "the munchies", a side effect of the drug. The Fleet attributed this storage to faulty food processors, but that is believed to be a coverup by loco weed smokers placed high up in the Fleet leadership. It is not known whether loco weed use contributed to Fat Apollo's weight issues, but it is considered likely.
New Caprica loco weed is an annual plant, easily distinguishable by its hand-shaped .... wait, the plant itself is never seen. For all that is known, it could be a perennial. It is known that, according to Admiral Adama, "it's good" (TRS: "Unfinished Business").
Also known as