Doxepin acts to change the balance of naturally occurring chemicals in the brain that regulate the transmission of nerve impulses between cells. Its action primarily increases the concentration of norepinephrine and serotonin (both chemicals that stimulate nerve cells) and, to a lesser extent, blocks the action of another brain chemical, acetylcholine. Although not technically a tricyclic antidepressant, doxepin shares most of the properties of these drugs, which include amitriptyline, clomipramine, desipramine, imipramine, nortriptyline, protriptyline, and trimipramine. Studies comparing doxepin with these other drugs have shown that doxepin is no more or less effective than other antidepressants of its type. Its choice for treatment is as much a function of physician preference as any other factor.
The therapeutic effects of doxepin, like other antidepressants, appear slowly. Maximum benefit is often not evident for at least two weeks after starting the drug. People taking doxepin should be aware of this and continue taking the drug as directed even if they do not see immediate improvement.
Recommended dosage
As with any antidepressant, doxepin must be carefully adjusted by the physician to produce the desired therapeutic effect. Doxepin is available as 10-mg, 25-mg, 50-mg, 75-mg, 100-mg, and 150-mg oral capsules as well as an oral concentrate solution containing 10 mg of drug in each milliliter of solution.
Therapy is usually started at 30 to 150 mg per day and gradually increased to 300 mg daily if needed. There is little evidence that doses above 300 mg daily provide any additional benefits. Amounts up to 150 mg may be taken as a single dose at bedtime to decrease daytime sleepiness. Doses of more than 150 mg per day should be divided into two or three doses and taken throughout the day.
In patients over age 60, therapy should be maintained at the low end of the dosing range and increased cautiously and with physician supervision. Patients with organic brain syndrome (psychiatric symptoms of dementia often seen in elderly patients) generally require daily doses of only 25 to 50 mg.
If the oral concentrate of doxepin is used, each dose should be diluted in at least 4 ounces (120 mL) of milk, orange, prune, tomato, pineapple, or grapefruit juice just before administration. Doxepin is not compatible with many carbonated beverages and should not be diluted in them.
Precautions
As with tricyclic antidepressants, doxepin should be used cautiously and with close physician supervision in people, especially the elderly, who have benign prostatic hypertrophy, urinary retention, and glaucoma, especially angle-closure glaucoma (the most severe form). Before starting treatment, people with these conditions should discuss the relative risks and benefits of treatment with their doctors to help determine if doxepin is the right antidepressant for them.
A common problem with antidepressants is sedation (drowsiness, lack of physical and mental alertness). This side effect is especially noticeable early in therapy. In most patients, sedation decreases or disappears entirely with time, but until then, patients taking doxepin should not perform hazardous activities requiring mental alertness or coordination. The sedative effect is increased when doxepin is taken with other central nervous system depressants, such as alcoholic beverages, sleeping medications, other sedatives, or antihistamines. It may be dangerous to take doxepin in combination with these substances. Doxepin may increase the possibility of having seizures. Patients should tell their physician if they have a history of seizures, including seizures brought on by the abuse of drugs or alcohol. These people should use doxepin only with caution and be closely monitored by their physician.
Doxepin may increase heart rate and stress on the heart. It may be dangerous for people with cardiovascular disease, especially those who have recently had a heart attack, to take this drug or other antidepressants in the same pharmacological class. In rare cases where patients with cardiovascular disease must receive doxepin, they should be monitored closely for cardiac rhythm disturbances and signs of cardiac stress or damage.
Doxepin should not be taken by nursing mothers because it is secreted into breast milk and may cause side effects in the nursing infant.
Side effects
Doxepin shares the side effects of tricyclic antidepressants. The most frequent of these are dry mouth, constipation, urinary retention, increased heart rate, sedation, irritability, dizziness, and decreased coordination. As with most side effects associated with tricyclic antidepressants, the intensity is highest at the beginning of therapy and tends to decrease with continued use.
Dry mouth, if severe to the point of causing difficulty speaking or swallowing, may be managed by dosage reduction or temporary discontinuation of the drug. Patients may also chew sugarless gum or suck on sugarless candy in order to increase the flow of saliva. Some artificial saliva products may give temporary relief.
Men with prostate enlargement who take doxepin may be especially likely to have problems with urinary retention. Symptoms include having difficulty starting a urine flow and more difficulty than usual passing urine. In most cases, urinary retention is managed with dose reduction or by switching to another type of antidepressant. In extreme cases, patients may require treatment with bethanechol, a drug that reverses this particular side effect. People who think they may be experiencing any side effects from this or any other medication should tell their physicians.
Interactions
Dangerously high blood pressure has resulted from the combination of antidepressants such as doxepin and members of another class of antidepressants known as monoamine oxidase (MAO) inhibitors. Because of this, doxepin should never be taken in combination with MAO inhibitors. Patients taking any MAO inhibitors, for example Nardil (phenelzinesulfate) or Parmate (tranylcypromine sulfate), should stop the MAO inhibitor then wait at least 14 days before starting doxepin or any tricyclic antidepressant. The same holds true when discontinuing doxepin and starting an MAO inhibitor.
Doxepin may decrease the blood pressure–lowering effects of clonidine. Patients who take both drugs should be monitored for loss of blood-pressure control and the dose of clonidine increased as needed.
American Society of Health-System Pharmacists. AHFS Drug Information 2002. Bethesda: American Society of Health-System Pharmacists, 2002.
DeVane, C. Lindsay, Pharm.D. "Drug Therapy for Mood Disorders." In Fundamentals of Monitoring Psychoactive Drug Therapy. Baltimore: Williams and Wilkins, 1990.
