Buprenorphine is the main active ingredient contained in both Subutex and Suboxone (Suboxone also contains naloxone) and, in higher doses, is used to treat opioid dependence. In lower doses, it is commonly used to treat moderate to chronic pain.
Buprenorphine is a derivative of the opium poppy plant just as other narcotics such as morphine and heroin are; however it is not quite as strong as these drugs. This is because buprenorphine is a partial opioid agonist, whereas drugs such as morphine, heroin, and oxycontin are full opioid agonists.
Basically, what this means, is that when an individual is administered buprenorphine, the chemical partially activates the opioid receptors in the individual’s brain, causing the person to feel slightly euphoric but nowhere near the intensity that they would feel if they were to take a full opioid agonist like heroin.
When an individual consumes heroin (by any number of methods), the drug fully activates the opioid receptors in the brain, causing an intense high feeling across the central nervous system of the body (assuming the dose is high enough).
Obviously, in small enough doses, no effect will be felt. This is why mental and physical dependence is so common with opiate drugs–because the body learns to love the feeling and eventually has a very hard time coping without it.