The common prescription brand name of the analgesic narcotic penazocine. Pentazocine is a strong pain killer, typically prescribed for moderate to severe pain. 50mg of pentazocine will give nearly the same pain-killing effect as 60mg of codeine. Like codeine, pentazocine produces a euphoric, peaceful state in the user, similar to a smaller dose of morphine. The typical opiate side effects apply- nausea, constipation, dizziness, fatigue, etc. are commonly reported. Fortunately, talwin isn't nearly as addictive as the typical opiate, which makes it ideal for patients experiencing chronic pain, and withdrawl symptoms are rarely reported. From a legal standpoint, pentazocine is a schedule IV narcotic in the United States, which means prescription only.

Recreationally, I highly recommend talwin as a hydro/oxycodone substitute. One .5mg pill is just enough for me. It takes at least 2 Tylenol-5 pills to match it. Also, Talwin seems to last a lot longer. I often find myself predicting a come-down too quickly and accidentally take too much, nearly renderring myself unconcious. Another benefit is the absence of acetaminophen, which is commonly combined with both hydrocodone and oxycodone. Ingesting large doses of acetaminophen is known to cause severe liver damage, especially in the case of frequent alcohol consumption. So if you pop vicodin regularly, I'd recommend giving the yellow pills a try, if only temporarily.

Source: http://www.rxlist.com