Whether you stored it away intentionally or forgot about an old stash, most pot consumers have wondered whether or not their weed has gone bad at some point. The short is answer is no — but that depends on your definition of "bad," as well as the method in which you stored it.
Technically, weed doesn't go bad in the sense that it will have a negative effect on the user. Antique pot won't poison you or have a dramatically different impact on your body. But its potency can certainly decrease or, at the least, be altered if not stored properly.
Marijuana plants have a number of chemical compounds known as cannabinoids present in their flowers. When these buds are dried out and subsequently heated, the cannabinoids are converted from biosynthetic acids into the weed's psychoactive substances in a process known as decarboxylation. The most notable transformation is Tetrahydrocannabinolic acid (THCA) into THC, which is the primary mind-altering property of cannabis.
According to the Colorado Pot Guide, THCA can be converted into a different cannabinoid called CBNA (and thus CBN) if the cannabis is exposed to excessive heat or sunlight. CBN is less psychoactive than THC, meaning its consumption does not get the user as high.
Additionally, Merry Jane pointed out that overly dried-out weed can be uncomfortable to smoke, as it tends to burn the throat when inhaled. Lastly, weed that's been overexposed to moisture can become moldy and have serious medical repercussions if consumed.
In most cases, though, smoking aged weed won't do anything other than giving you a lower high than usual. If stored right, marijuana can remain fresh and potent for months or even years. Keeping it a glass or ceramic jar in a cool, dark and dry place will do wonders. And if you really want to ensure maximum freshness, consider vacuum-sealing it.