Opioids are chemicals which act as analogues for the endorphins -- neurotransmitters with a wide variety of effects in the body including pain modulation, GI tract activity, and immune function.
Opioids bind to three types of receptors: mu, kappa, and delta opioid receptors. The three broad categories of opioid classification are (full) agonists, partial agonists, and antagonists.
Some patients may be unable to take drugs orally because of tumors, bowel obstruction, severe emesis, or severe dysphagia. In these cases, alternative routes of opioid administration are the transmucosal, transdermal, subcutaneous, rectal and intraspinal routes.

There are a variety of novel delivery methods available for opioid analgesics. One of the newer methods is the oral transmucosal delivery of medications, for which only certain agents are logical candidates. Fentanyl has an advantage in this regard due to is its high lipophilicity, which allows for rapid absorption across the oral mucosa into the blood and distribution into the CNS a process with a 3- to 5-minute half-life. This clearly explains why an oral transmucosal preparation can work as fast as IV morphine. In addition, fentanyl has no pharmacologically active metabolites. Morphine and Oxycodone are hydrophilic in nature, and therefore are not well suited for delivery across mucosal surfaces.

Adverse effects of opioids are varied, and usually are the limiting factor in therapy. There does not appear to be a "ceiling" dose, and therefore if an opioid is not effective at a particular dose, the solution is to increase the dosage until analgesia is achieved or side effects are encountered.