a-Absorption :

Most opioid analgesics are well absorbed when
given by subcutaneous, intramuscular, and oral routes. First -pass effect can
affect the opioid(morphine) and cause to give higher dose in oral more than
parentally . first-pass can predict the effect of the oral dose that given.
Certain analgesics such as codeine and oxycodone are effective orally because
they have reduced first-pass metabolism. Nasal insufflation of certain opioids
can result in rapid therapeutic blood levels by avoiding first-pass metabolism.
The latter can provide delivery of potent analgesics over days.

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B. Distribution

 The
uptake of opioids by different organs and tissues is a function of both
physiologic and chemical factors. Although all opioids bind to plasma proteins
with varying affinity, the drugs rapidly leave the blood compartment and
localize in highest concentrations in tissues that are highly perfused such as
the brain, lungs, liver, kidneys, and spleen. Drug concentrations in skeletal
muscle may be much lower, but this tissue serves as the main reservoir because
of its greater bulk.

C. Metabolism

 The
opioids are converted in large part to polar metabolites (mostly glucuronides),
which are then readily excreted by the kidneys. In contrast, approximately 10%
of morphine is metabolized to morphine- 6-glucuronide (M6G), an active
metabolite with analgesic potency four to six times that of its parent compound
. However, these relatively polar metabolites have limited ability to cross the
blood brain barrier and probably do not contribute significantly to the usual
CNS effects of morphine given acutely

D. Excretion

 Polar
metabolites, including glucuronide conjugates of opioid analgesics, are
excreted mainly in the urine. Small amounts of unchanged drug may also be found
in the urine. In addition, glucuronide conjugates are found in the bile, but
enterohepatic circulation represents only a small portion of the excretory
process.

 

 

Clinical Pharmacology Of The Opioids

A-Analgesia 

Severe, 
constant  pain is usually relieved
with opioid analgesics with high intrinsic activity, whereas sharp,
intermittent pain does not appear to be as effectively controlled.  Pain from cancer and sever illness must be
manage aggressively . Such conditions may need continuous management with
potent opioid . However, this should not be used as a barrier to providing
patients with the best possible care and quality of life. Studies shows that a
fixed admitting doses is better to relief the pain than 1 full dose of opioid .
New dosage forms of opioids that allow slower release of the drug are now
available, eg, sustained-release forms of morphine (MS Contin) and oxycodone
(OxyContin). Their purported advantage is a longer and more stable level of
analgesia.

 

B. Acute Pulmonary Edema

 The
relief produced by intravenous morphine in dyspnea from pulmonary edema
associated with left ventricular heart failure is remarkable. Proposed
mechanisms include reduced anxiety ( perception 
of shortness of breath) and reduced cardiac preload (reduced venous
tone) and afterload (decreased peripheral resistance). However, if respiratory
depression is a problem, furosemide may be preferred for the treatment of
pulmonary edema. On the other hand, morphine can be particularly useful when
treating painful myocardial ischemia with pulmonary edema.

C. Cough

 Can
obtained in lower dose than that which given for pain relief. However, in recent
years the use of opioid analgesics to allay cough has diminished largely
because a number of effective synthetic compounds have been developed that are
neither analgesic nor addictive .

D. Diarrhea

 Diarrhea from any disease or cause can be
managed with the opioid analgesics, only if the diarrhea is associated with
infection, such use must not substitute for appropriate chemotherapy. Crude
opium preparations (eg, paregoric) were used in the past to control diarrhea,
but now synthetic surrogates with more selective gastrointestinal effects and
few or no CNS effects, eg, diphenoxylate or loperamide, are used.

E. Shivering 

Although
all opioid agonists have some propensity to reduce shivering, meperidine is
reported to have the most pronounced ant shivering properties. Meperidine
apparently blocks shivering mainly through an action on subtypes of the ?
2  adrenoceptor. 

 

 

 

Adverse
effect of opioids

Nausea and vomiting: opioid
will stimulate the receptor of nausea and vomiting that present in
gastrointestinal tract also it will stimulate the center which present in
brain stem that will produce nausea and vomiting in patient. Some opioids
such as morphine and codeine cause more nausea than other opioids.
Drowsiness or sedation:
Opioids, and in particular morphine, are known to cause severe sedation
and drowsiness because it will affect the receptor in brain so patient who
use opioids will inter in state of drowsiness while he using them. People
taking opioids are thus advised to refrain from driving and operating
heavy machinery in order to avoid accidents.
Skin changes: urticaria may
develop and cause a skin rash which characterized by red, itchy, raised
bumps. Because of histamine released that stimulating by the giving of
opioid. Individuals may also experience flushing or cooling of the skin.
In the case of cooling, skin may appear cool and clammy and an individual
may shiver or even develop hypothermia.
Miosis: This describes the
formation of small, constricted pupils, similar to how pupils respond to
bright light.
Constipation: Opioids cause
sluggish peristaltic movements in the digestive tract. This will lead to
lose of intestinal movement or deacrease it that will cause a severe
constipation, especially in the case of long-term use.
Respiratory depression: The
breathing mechanism in response to a low blood oxygen level may be
suppressed. As blood oxygen falls and blood carbon dioxide rises, there is
an increase in drive for respiration. However, opioids adversely affect
the ventilatory response to hypoxemia, therefore increasing the risk for hypoventilation.
However, this occurs mostly in cases of opioids that are more potent and
taken in higher doses
Psychological effects: Opioids
give rise to a sense of euphoria and may also lead to hallucinations, delirium,
dizziness and confusion. There may be some amount of memory loss and
headache.
Changes in heart rate: Heart
rate may become either rapid or very slow. Some opioid users may also
develop postural hypotension or a severe fall in blood pressure on standing
up from a sitting or lying position.
Spasms: Some people may develop
spasms of the ureter and urinary retention or biliary colic and spasms of
the biliary tree.

 

 

 

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