The effect of agonist drug of opoid group have a lot of impact as listed:
Cough: The cough center in medulla is suppressed, leading to antitussive action. Pure agonists especially related with codeine have pronounced effect.
Nausea and Vomiting: The emesis is usually due to direct stimulation of chemoreceptor trigger zone (CTZ) of area postrema of the medulla. It is more commonly seen in ambulatory patients, less in recumbent, indicating may be the vestibular component is also involved.
Bradycardia: This is yet another effect of agonist drug .May be due to close proximity of vagal center in this area, bradycardia is seen due to some of the potent pure agonists like fentanyl. The bradycardia is dose dependent and can be blocked by atropine confirming vagal mediated response.
CVS: Pure agonists like morphine produce peripheral Vasodilatation, decreased peripheral resistance and cause inhibition of baroreceptor-mediated responses. The effect of agonist drug may be mediated through the vasomotor center. Also most of these drugs are responsible for release of histamine — again contributing in peripheral vasodilatation.
In the patients of ischaemic heart disease, especially in the attack of myocardial infarction, administration of morphine (pure agonists) produces beneficial effect of agonist drug due to following mechanisms:
a) Excellent quality of analgesia is produced, thus decreased release of catecholamines, leading to decreased oxygen demand.
b) As such, this effect of agonist drug can produce decreased oxygen consumption, decreased left ventricular end diastolic pressure and decreased cardiac work.
Pulmonary system: In addition to centrally induced respiratory depression, effect of agonist drug can induce the bronchospasm, especially in atopic individuals or known asthmatics, as a result of bronchoconstrictory action due to released histamine.
In the patients with chronic obstructive airway diseases (COPD), these drugs should be given with great caution, ifpossible avoided completely.
GIT: The effect of agonist drug is that of decreased motility of stomach, decreased stomach emptying, but also decreasing acid secretion due to local receptor mediated inhibition of parietal cells of stomach.
Due to inhibition of myenteric plexuses ofAuerbach’s and Meissner’s, decreased motility of body of the viscus be it small or large intestinal walls, and sphincteral constriction causing constipation.
As a result of effect of agonist drug , the standard dictum given in surgical books: “Opioids like morphine are contraindicated in acute abdomen as they can actually mask the progress of the condition by causing decreased motility of visoera with sphincteral spasm”.
Biliary system: Again due to spasm of sphincter of Oddi, it becomes clinically evident biliary spasm and cholestatic jaundice.
Urinary bladder and ureter: This effect of agonist drug is same as GIT. The detrusor is relaxed and sphincteral spasm is produced leading to urinary retention.
Skin: Pruritus, almost all the pure agonists are implicated in pruritus, especially synthetic opioids like fentanyl and sufentanil. Typically the complaint is sensation of itching in nose. The flushing of skin of head, neck and face is also frequently seen as effect of agonist drug .