Opioids ago since evidence shows that opium use

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Opioids are the mainstay of the
treatment of moderate to severe pain. The use of opioids dates back to
centuries ago since evidence shows that opium use was present even before
civilization, approximately 5000 years ago. However, physicians have always
tried to balance the benefits associated with opioid use with their immense
side effects and dependence. There are various forms of opioids that are still
illegal in the United States such as heroin. The legal forms of opioids can be
greatly categorized as strong, moderate, or weak. Strong opioids include
morphine, pethidine, fentanyl, and sufentanil. On the other hand, intermediate
or moderate opioids include drugs such as buprenorphine while codeine is a weak
opioid. The prolonged use of any opioid can cause dependence and if the drug is
withdrawn abruptly, withdrawal symptoms ensue. Therefore, it is important to
administer these drugs with a lot of caution in order to avoid any
complications associated with their misuse. In case a patient has an opioid
overdose, naloxone or naltrexone should be administered since antagonize the
effects of opioids in the human body (Casy
& Parfitt, 2013). This paper discusses the pharmacokinetics and
pharmacodynamics of the different types of opioid analgesics.

Mechanism of Action of Opioid Analgesics

            The mechanism of action of opioid analgesics is overly
similar. They act by binding to the kappa, delta, nociceptin, and mu opioid
receptors. These receptors are distributed in the central nervous system and
they are mainly concentrated at the thalamus, cerebral cortex, grey area, and
in the substantia gelatinosa of the spinal cord. The receptors can also be
found in peripheral nerve terminals and in other organs. These receptors are
attached to the inhibitory G-proteins and once the receptors are activated, the
G-proteins close the calcium channels and facilitate the influx of potassium
which causes hyperpolarization. Hyperpolarization decreases neuronal
excitability hence reducing the transmission of nociceptive chemical impulses
and this causes analgesia (Trescot,

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            Morphine is the most potent opioid drug and it is used in
the management of severe pain. It is derived from the poppy plant and the
seedpods of the poppy plant are processed to form the alkaloids. Ammonium
hydroxide is then used to precipitate morphine from other alkaloids,
afterwards, the final product is purified. Morphine can either be taken orally
or given through the intravenous route. Morphine can be absorbed in the
gastrointestinal tract although it undergoes hepatic first-bypass in the liver
and this reduces its bioavailability to approximately 25%. Its metabolism in
the liver yields morphine-3-glucoronide which does not have any pharmacological
effects. However, morphine-6-glucoronide is another metabolite that is formed
after hepatic metabolism, this metabolism is highly potent. Morphine has a
large volume of distribution and most of the drug remains unbound to plasma
proteins. Morphine is cleared primarily through the liver although 30%
undergoes renal clearance. This extra hepatic clearance is crucial in patients
with liver dysfunction (Trescot,

            Morphine is indicated for the treatment of acute pain
such as that experienced in myocardial infarction, injuries, and it can also be
used after a surgical procedure. Also, in patients with terminal illnesses such
as cancer, morphine can be used as a long-term analgesic. However, such
patients should be carefully monitored to prevent them from developing
dependence and addiction (Jamison
& Mao, 2015, July).




            Pethidine is sometimes referred to as meperidine and it
is a less potent opioid analgesic. Pethidine is a synthetic opioid unlike
morphine which is naturally occurring. Just like morphine, it also undergoes
extensive hepatic first-bypass and its oral availability is between 45% and
75%. However, its oral absorption I quite slow and it reaches its peak in
around 2 hours. The drug can either be administered orally or intramuscularly
although the parenteral route is more erratic. It is primarily metabolized in
the liver and at has a half-life of 3 hours, in patients with liver
dysfunction, this half-life is often prolonged. The drug metabolites can be
excreted through the kidney, thus any renal insufficiency can cause toxicity (Trescot, 2016).

            Under its pharmacodynamics, pethidine is approximately 10
times less potent than morphine in regards to analgesia. Following oral
administration, the peak is reached within 1 to 2 hours while after parenteral
administration the peak is achieved in less than 60 minutes. The analgesic
effect is, however, short-lived as it does not last for more than 3 hours.
Pethidine is known to cause excessive central nervous system excitation which
can cause tremors and seizures. Just like all other opioids, pethidine causes
euphoria, respiratory depression, pupillary dilatation, and nausea and
vomiting. However, pethidine can cause tachycardia unlike other narcotics and it
also stimulate the release of histamine. When given in large doses, pethidine
can decrease myocardial contractility thus causing hypotension (Trang et al., 2015).  


            This is a synthetic opioid analgesic. However, it is up
to 100 times more potent than morphine. It is mainly used to treat severe pain
and it can also be used in patients who have become tolerant to other narcotic
analgesics. Fentanyl has many routes of administration which are; oral,
transdermal, intravenous, intramuscular, and sublingual routes of drug
administration. Fentanyl predominantly binds the mu-receptors although it can
also bind to the kappa and delta-opioid receptors. Its principal
pharmacological are exerted on the central nervous system causing euphoria,
dysphoria, sedation, analgesia, drowsiness, pupillary constriction, and respiratory
depression (Casy &
Parfitt, 2013).

            Following transdermal administration its bioavailability
is 92% while buccal administration produces a 50% drug bioavailability.
Approximately 80 to 85% of the drug is bound to plasma proteins and its volume
of distribution is between 3L to 8L per kilogram although the volume is reduced
in patients with hepatic impairment. It is primarily metabolized in the liver
and cytochrome P450 is the enzyme that catalyzes this reaction. Fentanyl is
excreted through urine and almost 75% of the intravenously administered
fentanyl will be eliminated in urine within 72 hours. It has a half-life of 7
hours (Casy & Parfitt, 2013).


            This is a short-acting synthetic opioid analgesic with a
half-life of 1 to 20 minutes. It mainly binds to the mu-opioid receptors. The
drug is administered intravenously and it has a rapid onset and its effects and
related side effects are dose dependent. Approximately 70% of the drug binds to
plasma proteins and its volume of distribution is 350ml per kilogram in adults.
It is metabolized by non-specific blood and tissue esterases and this yields
its principal metabolite, remifentanil acid. This metabolite is excreted
through the kidneys at the rate of 40ml/minute/kilogram in a healthy adult (Pasternak & Pan, 2013).

Categories: Management


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