Antidepressants: Physiological effects

Last modified: Thursday, 25. December 2008 - 5:52 am


The physiological action of antidepressants is not fully understood. However, they are thought to influence the metabolism, reuptake, or selective receptor antagonism of the neurotransmitters serotonin and norepinephrine. The MAOIs cause an increase of the neurotransmitters norepinephrine, serotonin, and dopamine in the brain by inhibiting their breakdown. The SSRIs prevent the reabsorbtion of serotonin, one of the 50-odd neurotransmitters in the brain.

Studies show little difference in the effects of the various antidepressants, but some individual patients appear to do better on one drug than another. In the search for the most effective drug for a particular patient, the physician may try various antidepressant drugs or even try some in combination.

Antidepressants do not cause addiction, which is usually evident by the build-up of tolerance (needing increasing amounts of a drug to achieve the initial effect). Withdrawal symptoms do not occur when the drug is discontinued. However, gradually decreasing the dosage will give the brain a chance to take over or function on its own without the drug, and help avoid some physiological symptoms such as the flu, headache, or nausea that sometimes occur. This gradual cutting back on the amount of drug can assist the physician in prescribing if the patient’s symptoms return.

Harmful side effects

Patients are cautioned not to operate machinery until they know how the drug affects them personally (although antidepressants are not known to cause impairment in this way). Side effects can include agitation, insomnia or drowsiness, and thirst. Serious side effects are not expected with these drugs.

Because of their selective action on serotonin, the manufacturer of SSRIs claim there are fewer side effects from this class of drug than those of the tricyclics and MAOIs. These other drugs affect many neurotransmitters in the brain and therefore result in many side effects. However, the SSRIs can cause stomach upset, insomnia, and anxiety.

For the most part, antidepressants are considered safe. However, they may interact with other drugs taken by the patient — and the physician prescribing one of these drugs must take this into consideration. Additionally, some medical problems already under treatment may increase in severity in the presence of an antidepressant.

Patients suffering from bipolar disorder whose high periods exhibit anger, paranoia, or irritability may exhibit these symptoms when taking Prozac if they have not been first stabilized with another medication such as lithium.

The adverse effects of antidepressants will vary by drug and may include some or several of the following: insomnia, tremor, gastrointestinal symptoms, rashes, sexual dysfunction, anxiety, dizziness, dry mouth, sweating, sleepiness, blurred vision, constipation, aggravation of psychosis, weight gain, increased appetite, arrhythmias (disturbed rhythm of the heartbeat), hypertension or postural hypotension, nausea, and, with high doses, seizures.

It is unusual for the MAOIs to have a toxic effect. However, when it does occur the symptoms are severe and include dulled consciousness, seizures, shock, and hyperthermia (elevated temperature). Although some reactions with SSRIs — and even some fatalities — have occurred with overdoses, they are extremely rare. A reaction may be due to the combining of the SSRIs with other drugs.

Side effects of St. John’s wort include headache, gastrointestinal discomfort, sun sensitivity, and dry mouth. Some of the effects of combining the herb with other drugs are very serious. For example, St. John’s wort lessens the effectiveness of HIV drugs, as well as some drugs that are essential after a transplant operation and heart drugs (such as digoxin). It is essential that anyone desiring to take the herb consult with their doctor.

Overdoses of the antidepressants can result in death. However, the effects of overdosing will vary according to which drug is taken.

Long-term health effects

Long-term health effects from antidepressants include the increased risk of deliberate self-harm (DSH), which may occur more with the SRRIs than the tricyclic antidepressants. The self-harm may occur by overdosing with the prescribed drug, but is more frequently by other means.

Long-term use of the SSRIs has been found to increase the risk of gastrointestinal (GI) bleeding, especially in the elderly and in those who have previously experienced bleeding. In addition, although preliminary studies showed a decreased risk of heart attack for those using SSRIs, an increased risk for arrhythmias (irregular heart beats) and heart attacks has been observed in those taking the tricyclics. These older tricyclic antidepressants may disrupt the heartbeat, possibly resulting in death. Therefore, it is essential that physicians test patients prior to and during treatment with tricyclics.

Tricyclics may also pose a threat to vision by causing dry eyes, blurred vision, and even vision loss when narrow-angle glaucoma is present. Also, tricyclics may result in dry mouth that can lead to dental cavities, reduced urine output, and constipation. Although not fully understood, weight gain is also an effect. Whether a result of the drug or of the illness, sexual drive is decreased.

MAOIs frequently are not the first choice when an antidepressant is needed. This is because their side effects include sedation, dizziness, insomnia, sexual dysfunction, rapid heartbeat, constipation, and agitation. Severe sudden elevation of blood pressure, especially when the patient eats aged meats and cheeses, or takes some over-the-counter cold medications, poses a serious threat of a stroke or other symptoms including headache, vomiting, and palpitations.

Studies of the long term-health effects of St. John’s wort are in progress. Avoidance of the herb by pregnant and lactating women is strongly recommended.


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