Opioid rectal administration

Opioids maybe administered in a variety of routes including oral (tablet and liquid), sublingual, rectal, transdermal, transmucosal, intravenous, subcutaneous, and intraspinal. While the oral and transdermal routes are most common, the method of administration is based on patient needs (severity of pain) and characteristics (swallowing difficulty and preference). Oral opioids have an onset of effect of 45 minutes, so intravenous or subcutaneous administration maybe preferred if more rapid relief is desired. Intramuscular injections are not recommended because of pain at the injection site and wide fluctuations in drug absorption and peak plasma concentrations achieved. More invasive routes of administration such as PCA and intraspinal (epidural and intrathecal) are primarily used postoperatively, but may also be used in refractory chronic pain situations. PCA delivers a self-administered dose via an infusion pump with a preprogrammed dose, minimum dosing interval, and maximum hourly dose. Morphine, fentanyl, and hydromorphone are commonly administered via PCA pumps by the intravenous route, but less frequently by the subcutaneous or epidural route. 

Opioids are administered in several ways. Opium was most commonly taken recreationally by smoking, but intravenous administration has become most common since the isolation of opium alkaloids and invention of the hypodermic needle. The development of heroin from morphine at the turn of the twentieth century led to more intense euphoric effects and greater risk for addiction. Heroin may also be snorted, or it can be smoked when added to a medium such as tobacco. Medically, opioids are commonly given through oral, subcutaneous, intravenous, transdermal, or rectal routes. 

Some opioids are available in rectal formulations, and administration via this route avoids the first-pass effect. If liver metabolism is sufficiently impaired, administration via this route would theoretically cause less of an increase in peak opioid levels. 

Opioids have traditionally been given intramuscularly and intravenously. Other methods of administration are oral, subcutaneous, rectal, intrathecal, and extradural. Novel routes include intranasal, inhalational, intra-articu-lar, and transdermal. 

The more novel routes of administration of opioids, including oral, nasal, rectal, transdermal, spinal, and by patient-controlled methods, have been outlined (SEDA-17, 78). Oral transmucosal fentanyl administration, avoiding first-pass metabolism, produces analgesia and sedation in both adults and children undergoing short, painful outpatient procedures. The quality of analgesia is good, and the adverse effects are those typical of the opioids. 

The availability of new routes of administration have led to increased utility and decreased opioid adverse drug reaction risk. Epidural and intrathecal administration through spinal catheters produces adequate regional analgesia at relatively low total doses compared with intravenous or oral routes. As such, spinal administration can thus minimize somnolence, nausea, vomiting, and respiratory depression associated with these medications. Other alternative routes include intranasal administration of butorphanol, and rectal and transdermal administration of fentanyl [28]. Availability of such options provides not only a decreased risk of adverse reactions, but also more comfortable measures for patients who would otherwise require continued intravenous administration, or for those who are unable to receive oral medication [28,29]. 

Ease of use morphine and all the other injectable opioids are very easy to use. One major advantage of morphine that leads to its ease of use is the multiple routes of administration (IV, IM, SQ, IT epidural, intraventricular, rectal, oral, and inhalational). Morphine can easily be titrated to sedation levels, pain scores, and respiratory rate. 

Methadone is a potent synthetic opioid analgesic, structurally unrelated to any of the opium-derived alkaloids. It is a highly lipophilic, basic drug (pKa 9.2) available as a hydrochloride powder formulation that can be reconstituted for oral, rectal, or parenteral administration. Methadone was developed in Germany in 1942 as a synthetic substitute for morphine, and has been approved and widely employed for opioid detoxification maintenance as well as acute and chronic pain management. 

---