Morphine Side Effects

Constipation

Like loperamide and other opioids, morphine acts on the myenteric plexus in the intestinal tract, reducing gut motility, causing constipation. The gastrointestinal effects of morphine are mediated primarily by μ-opioid receptors in the bowel. By inhibiting gastric emptying and reducing propulsive peristalsis of the intestine, morphine decreases the rate of intestinal transit. Reduction in gut secretion and increases in intestinal fluid absorption also contribute to the constipating effect. Opioids also may act on the gut indirectly through tonic gut spasms after inhibition of nitric oxide generation. This effect was shown in animals when a nitric oxide precursor, L-Arginine, reversed morphine-induced changes in gut motility.

Addiction

In controlled studies comparing the physiological and subjective effects of injected heroin and morphine in individuals formerly addicted to opiates, subjects showed no preference for one drug over the other. Equipotent, injected doses had comparable action courses, with no difference in subjects' self-rated feelings of euphoria, ambition, nervousness, relaxation, drowsiness, or sleepiness. In these studies, rats with a morphine "addiction" overcome their addiction themselves when placed in decent living environments with enough space, good food, companionship, areas for exercise, and areas for privacy. More recent research has shown that an enriched environment may decrease morphine addiction in mice.

Morphine is a potentially highly addictive substance. It can cause psychological dependence and physical dependence as well as tolerance, with an addiction potential identical to that of heroin. When used illicitly, a very serious narcotic habit can develop in a matter of weeks, whereas iatrogenic morphine addiction rates have, according to a number of studies, remained nearly constant at one case in 150 to 200 for at least two centuries. In the presence of pain and the other disorders for which morphine is indicated, a combination of psychological and physiological factors tend to prevent true addiction from developing, although physical dependence and tolerance will develop with protracted opioid therapy. These two factors do ''not'' add up to addiction without psychological dependence which manifests primarily as a morbid seek orientation for the drug.

Tolerance

Tolerance to the analgesic effects of morphine is fairly rapid. There are several hypotheses about how tolerance develops, including opioid receptor phosphorylation (which would change the receptor conformation), functional decoupling of receptors from G-proteins (leading to receptor desensitization), mu-opioid receptor internalization and/or receptor down-regulation (reducing the number of available receptors for morphine to act on), and upregulation of the cAMP pathway (a counterregulatory mechanism to opioid effects) (For a review of these processes, see Koch and Hollt.) CCK might mediate some counter-regulatory pathways responsible of opioid tolerance. CCK-antagonist drugs, specifically proglumide, have been shown to slow the development of tolerance to morphine.

Withdrawal

The withdrawal symptoms associated with morphine addiction are usually experienced shortly before the time of the next scheduled dose, sometimes within as early as a few hours (usually between 6–12 hours) after the last administration. Early symptoms include watery eyes, insomnia, diarrhea, runny nose, yawning, dysphoria, sweating and in some cases a strong drug craving. Severe headache, restlessness, irritability, loss of appetite, body aches, severe abdominal pain, nausea and vomiting, tremors, and even stronger and more intense drug craving appear as the syndrome progresses. Severe depression and vomiting are very common. During the acute withdrawal period systolic and diastolic blood pressure increase, usually beyond pre-morphine levels, and heart rate increases, which could potentially cause a heart attack, blood clot, or stroke.

Chills or cold flashes with goose bumps ("cold turkey") alternating with flushing (hot flashes), kicking movements of the legs ("kicking the habit") and excessive sweating are also characteristic symptoms. Severe pains in the bones and muscles of the back and extremities occur, as do muscle spasms. At any point during this process, a suitable narcotic can be administered that will dramatically reverse the withdrawal symptoms. Major withdrawal symptoms peak between 48 and 96 hours after the last dose and subside after about 8 to 12 days. Sudden withdrawal by heavily dependent users who are in poor health is very rarely fatal. Morphine withdrawal is considered less dangerous than alcohol, barbiturate, or benzodiazepine withdrawal.

The psychological dependence associated with morphine addiction is complex and protracted. Long after the physical need for morphine has passed, the addict will usually continue to think and talk about the use of morphine (or other drugs) and feel strange or overwhelmed coping with daily activities without being under the influence of morphine. Psychological withdrawal from morphine is a very long and painful process. Addicts often suffer severe depression, anxiety, insomnia, mood swings, amnesia (forgetfulness), low self-esteem, confusion, paranoia, and other psychological disorders. The psychological dependence on morphine can, and usually does, last a lifetime. There is a high probability that relapse will occur after morphine withdrawal when neither the physical environment nor the behavioral motivators that contributed to the abuse have been altered. Testimony to morphine's addictive and reinforcing nature is its relapse rate. Abusers of morphine (and heroin), have one of the highest relapse rates among all drug users.

Hepatitis C

Researchers at the University of Pennsylvania have demonstrated that morphine withdrawal complicates hepatitis C by suppressing IFN-alpha-mediated immunity and enhancing virus replication. Hepatitis C virus (HCV) is common among intravenous drug users. This high association has piqued interest in determining the effects of drug abuse, specifically morphine and heroin, on progression of the disease. The discovery of such an association would impact treatment of both HCV infection and drug abuse.

Overdose

A morphine overdose occurs by intentionally or accidentally taking too much of it. A large overdose can cause asphyxia and death by respiratory depression if the person does not get medical attention or an antidote (Naloxone) immediately.

Treatments include administration of activated charcoal, intravenous fluids, laxatives and naloxone. The latter is an antidote to reverse the effect of the poison. Multiple doses of it may be needed.

Although it has previously been thought that morphine was contraindicated in acute pancreatitis, a review of the literature shows no evidence for this.

Performance

Most reviews conclude that opioids produce minimal impairment of human performance on tests of sensory, motor, or attentional abilities. However, recent studies have been able to show some impairments caused by morphine, which is not surprising given that morphine is a central nervous system depressant. Morphine has resulted in impaired functioning on critical flicker frequency (a measure of overall CNS arousal) and impaired performance on the Maddox Wing test (a measure of deviation of the visual axes of the eyes). Few studies have investigated the effects of morphine on motor abilities; a high dose of morphine can impair finger tapping and the ability to maintain a low constant level of isometric force (i.e. fine motor control is impaired), though no studies have shown a correlation between morphine and gross motor abilities.

In terms of cognitive abilities, one study has shown that morphine may have a negative impact on anterograde and retrograde memory, but these effects are minimal and are transient. Overall, it seems that acute doses of opioids in non-tolerant subjects produce minor effects in some sensory and motor abilities, and perhaps also in attention and cognition. It is likely that the effects of morphine will be more pronounced in opioid-naive subjects than chronic opioid users.

In chronic opioid users, such as those on Chronic Opioid Analgesic Therapy (COAT) for managing severe, chronic pain, behavioural testing has shown normal functioning on perception, cognition, coordination and behaviour in most cases. One recent study analysed COAT patients in order to determine whether they were able to safely operate a motor vehicle. The findings from this study suggest that stable opioid use does not significantly impair abilities inherent in driving (this includes physical, cognitive and perceptual skills). COAT patients showed rapid completion of tasks which require speed of responding for successful performance (e.g. Rey Complex Figure Test) but made more errors than controls. COAT patients showed no deficits in visual-spatial perception and organization (as shown in the WAIS-R Block Design Test) but did show impaired immediate and short-term visual memory (as shown on the Rey Complex Figure Test – Recall). These patients showed no impairments in higher order cognitive abilities (i.e. Planning). COAT patients appeared to have difficulty following instructions and showed a propensity towards impulsive behaviour, yet this did not reach statistical significance. Importantly, this study reveals that COAT patients have no domain-specific deficits, which supports the notion that chronic opioid use has minor effects on psychomotor, cognitive, or neuropsychological functioning.

It is difficult to study the performance effects of morphine without considering why a person is taking morphine. Opioid-naive subjects are volunteers in a pain-free state. However, most chronic-users of morphine use it to manage pain. Pain is a stressor and so it can confound performance results, especially on tests that require a large degree of concentration. Pain is also variable, and will vary over time and from person to person. It is unclear to what extent the stress of pain may cause impairments, and it is also unclear whether morphine is potentiating or attenuating these impairments.