Doxepin Hydrochloride

Introduction Overview

Generic Name

doxepin hydrochloride

Brand Name

Sinequan, Silenor

Dosage Form

oral solution (concentrate)

Drug Class

Miscellaneous anxiolytics, sedatives and hypnotics
Tricyclic antidepressants

Pharmacokinetics

Absorption: Doxepin hydrochloride is administered orally primarily as an oral solution concentrate or capsules/tablets, with variable bioavailability due to first-pass metabolism. Oral absorption of doxepin is relatively efficient but affected by hepatic metabolism, which can result in individual variability in plasma levels. The onset of action for anxiolytic and sedative effects is relatively rapid, with clinical effects often observed before antidepressant benefits, which may take several weeks to manifest.

Distribution: Doxepin is extensively distributed throughout the body tissues, exhibiting high lipid solubility which facilitates penetration across the blood-brain barrier, critical for its central nervous system effects. Plasma protein binding is substantial, primarily to albumin and alpha-1 acid glycoprotein, influencing both drug disposition and potential drug interactions. The volume of distribution is large, indicative of extensive tissue binding.

Metabolism: Doxepin is primarily metabolized by hepatic enzymes, predominantly cytochrome P450 isoenzyme CYP2D6, with minor contributions from CYP1A2 and CYP3A4 pathways. The metabolism results in the formation of active and inactive metabolites, with an isomeric mixture component affecting pharmacological activity. Genetic polymorphisms in CYP2D6 can significantly alter plasma concentrations, with poor metabolizers exhibiting elevated drug levels and increased risk for toxicity. Concomitant administration of CYP2D6 inhibitors such as SSRIs (e.g., fluoxetine, paroxetine) can potentiate doxepin plasma levels, requiring careful monitoring and possible dose adjustments.

Elimination: Doxepin and its metabolites are primarily excreted via renal pathways. The elimination half-life ranges considerably due to individual metabolic differences but generally falls between 8 to 24 hours, allowing for once or twice daily dosing regimens depending on clinical indication and patient tolerance. Renal impairment may affect clearance, necessitating dose titration particularly in elderly patients. The drug does not undergo significant elimination via dialysis or other extracorporeal removal methods.

Dosage Forms

Available Dosage Forms: Doxepin hydrochloride is supplied as an oral solution concentrate containing 10 mg/mL doxepin hydrochloride. Packaging typically includes 120 mL bottles with an accompanying calibrated dropper for measurement of 5 mg, 10 mg, 15 mg, 20 mg, and 25 mg doses. Oral tablets and capsules are also available but were not detailed in the provided data.

Dosages for Adults and Adolescents: Recommended initial dosing for adults with mild to moderate illness starts at 75 mg per day, which may be incrementally adjusted based on clinical response and tolerability, with optimal doses generally ranging from 75 to 150 mg daily. More severe cases may require gradual titration up to a maximum of 300 mg daily, though doses above 300 mg rarely provide additional benefit. The total daily dose may be administered once daily (preferably at bedtime to utilize sedative effects) or divided over the day, with a maximum of 150 mg recommended for once-daily dosing.

Specific Patient Populations: Elderly patients should initiate therapy at lower doses due to altered pharmacokinetics related to decreased hepatic, renal, or cardiac function, coupled with increased sensitivity to sedating effects, requiring close observation. Pediatric use of doxepin oral solution is not recommended for children under 12 years owing to lack of established safety and efficacy. Adjustments for hepatic or renal impairment are advised based on clinical judgment. Patients with glaucoma or urinary retention should avoid doxepin due to anticholinergic effects.

Mechanism Of Action

Doxepin hydrochloride is a dibenzoxepin derivative belonging to the tricyclic antidepressant class, exhibiting multiple mechanisms that contribute to its clinical effects in depression, anxiety, and related disorders. Its primary pharmacologic action is believed to be the inhibition of the reuptake of norepinephrine and serotonin (5-HT) at presynaptic nerve terminals, thereby increasing synaptic concentrations of these neurotransmitters and enhancing neurotransmission in central adrenergic and serotonergic pathways. This action contributes to the mood-elevating and anxiolytic effects observed in patients.

In addition to its monoamine reuptake inhibition, doxepin exhibits antagonistic activity at various receptor sites including histaminergic (H1), muscarinic cholinergic, and adrenergic receptors, which contributes to its sedative, anticholinergic, and antihistaminic properties. The potent H1 receptor antagonism accounts for prominent sedation, making doxepin effective as a hypnotic agent at lower doses. Its anticholinergic activity can produce side effects such as dry mouth, blurred vision, and urinary retention.

Animal studies have demonstrated that doxepin may potentiate norepinephrine responses at higher doses and exert antiserotonin actions on smooth muscle; however, these effects appear less pronounced or variable in humans. Importantly, unlike monoamine oxidase inhibitors, doxepin does not inhibit monoamine oxidase and is not a central nervous system stimulant, differentiating it mechanistically from other antidepressants.

Clinically, the combination of neurotransmitter reuptake inhibition and receptor antagonism yields a broad therapeutic profile targeting depressive and anxiety symptoms, with the sedative effects beneficial for patients with concomitant insomnia. The unique receptor interaction profile distinguishes doxepin from selective serotonin reuptake inhibitors (SSRIs) and other antidepressants, often leading to differing efficacy and side effect profiles. Ongoing research explores the contributions of receptor interactions to clinical outcomes and tolerability.

Pharmacological Actions

Inhibition of norepinephrine and serotonin reuptake at presynaptic nerve terminals resulting in enhanced monoaminergic neurotransmission.
Potent antagonism of histamine H1 receptors producing sedative, antihistaminic effects.
Anticholinergic activity via muscarinic receptor blockade causing effects such as dry mouth and urinary retention.
Adrenergic alpha-1 receptor antagonism contributing to side effects like orthostatic hypotension.
Anxiolytic and antidepressant effects mediated through modulation of central neurotransmitter systems.
Minimal euphoric potential and low risk of physical dependence or tolerance.
Possible potentiation of norepinephrine responses at higher doses as demonstrated in animal models.
Lack of monoamine oxidase inhibition distinguishes it mechanistically from MAO inhibitors.

Therapeutic Or Clinical Uses

Treatment of psychoneurotic patients experiencing depression and/or anxiety disorders.
Management of depression and/or anxiety symptoms associated with alcoholism (contraindicated concomitant alcohol use).
Treatment of depression and/or anxiety linked to organic diseases, with consideration for drug interactions.
Management of psychotic depressive disorders with anxiety components, including involutional depression and manic-depressive disorders.
Alleviation of target psychoneurotic symptoms such as anxiety, tension, somatic complaints, guilt, lack of energy, fear, apprehension, and sleep disturbances.
Sedative usage for patients experiencing significant insomnia related to depressive or anxiolytic symptoms.
Off-label or adjunct therapy in elderly patients for depression and anxiety, with cautious use.
Use is not recommended in pediatric patients under 12 years due to insufficient evidence of safety and efficacy.

Side Or Adverse Effects

Dry mouth, blurred vision, constipation, and urinary retention secondary to anticholinergic effects.
Drowsiness, sedation, and CNS depression, commonly observed especially during initiation.
Confusion, disorientation, hallucinations, paresthesias, ataxia, extrapyramidal symptoms, seizures, tardive dyskinesia, and tremor infrequently reported.
Cardiovascular effects such as hypotension, hypertension, and tachycardia.
Allergic reactions including skin rash, edema, photosensitivity, and pruritus.
Hematologic abnormalities like eosinophilia and rare bone marrow suppression presenting as agranulocytosis, leukopenia, thrombocytopenia, and purpura.
Gastrointestinal complaints including nausea, vomiting, indigestion, taste disturbances, diarrhea, anorexia, and aphthous stomatitis.
Endocrine disturbances such as altered libido, gynecomastia, breast enlargement, galactorrhea, blood sugar fluctuations, and syndrome of inappropriate antidiuretic hormone secretion (SIADH).
Other adverse effects including dizziness, tinnitus, weight gain, sweating, chills, fatigue, weakness, flushing, jaundice, alopecia, headache, asthma exacerbation, and hyperpyrexia.
Potential withdrawal symptoms upon abrupt cessation, characterized by nonspecific symptoms; not associated with addiction.
Risk of exacerbation or emergence of psychotic or manic symptoms requiring dosage adjustment or additional therapy.

Precautions

Patients and caregivers should be informed about the risks and benefits of therapy and counseled on recognizing signs of worsening depression or suicidality.
Monitor patients closely for emergence of anxiety, agitation, panic attacks, irritability, hostility, impulsivity, hypomania, mania, and unusual behavior changes, especially early in treatment or dose adjustments.
Patients with a history or risk of bipolar disorder should be screened prior to initiating therapy to avoid precipitating manic episodes.
Use with caution in patients susceptible to angle-closure glaucoma due to potential pupillary dilation caused by the drug.
Prescribers should advise patients to avoid operating heavy machinery or driving until they understand how doxepin affects them, due to possible sedation.
Concomitant use with alcohol increases risk of CNS depression and toxicity; alcohol consumption should be avoided.
Cautious dose initiation and titration are recommended in elderly patients due to increased susceptibility to side effects and altered metabolism.
Abrupt discontinuation should be avoided to prevent withdrawal symptoms; gradual tapering is advised.
Patients taking CYP2D6 inhibitors or known poor metabolizers require careful dose adjustment and monitoring due to elevated plasma concentrations.
Regular monitoring may be necessary when administered with drugs metabolized or affecting cytochrome P450 to avoid interactions.
Patients with narrow-angle glaucoma or urinary retention should avoid doxepin administration.
Pregnancy and lactation: Safety in pregnancy has not been established; potential neonatal effects have been reported.
Inform patients about potential allergic reactions and to report any rash or hypersensitivity symptoms immediately.
Psychiatric follow-up is essential, especially in cases of overdose or manifestation of worsening psychiatric symptoms.

Warnings

Increase risk of suicidal thoughts and behaviors in children, adolescents, and young adults during initial months of antidepressant therapy; close monitoring is required.
Patients with major depressive disorder or other psychiatric conditions may experience worsening of symptoms or emergence of suicidality regardless of treatment.
Potential emergence of anxiety, agitation, panic attacks, insomnia, irritability, hostility, aggressiveness, impulsivity, akathisia, hypomania, and mania during treatment.
Not approved for use in pediatric patients under 12 years; use in this population not established as safe.
Can precipitate angle-closure glaucoma in susceptible individuals due to pupillary dilation.
Use cautiously in elderly patients; start with low doses and monitor for sedation, confusion, and orthostatic hypotension.
Risk of life-threatening arrhythmias in overdose; requires immediate medical attention and cardiac monitoring.
Concomitant monoamine oxidase inhibitor (MAOI) use is contraindicated due to risk of serious reactions.
Use with caution in patients with cardiovascular disease, history of seizures, or urinary retention.
Withdrawal symptoms can occur after abrupt discontinuation; gradual tapering recommended.
Possibility of increased toxicity when combined with drugs that inhibit CYP2D6 enzyme.
May cause sedation leading to impaired ability to perform tasks requiring alertness.
Prescriptions should be limited to smallest amount consistent with good patient management to reduce overdose risk.

Contraindication

Hypersensitivity to doxepin or any component of the formulation.
Known cross-sensitivity with other dibenzoxepines or tricyclic antidepressants.
Patients with narrow-angle glaucoma or predisposition to angle-closure glaucoma.
Patients with urinary retention disorders.
Concomitant or recent (within 14 days) use of monoamine oxidase inhibitors (MAOIs).
Use in pediatric patients under 12 years due to lack of safety and efficacy data.
Patients with recovering from acute myocardial infarction or those with unstable cardiac conditions (due to risk of arrhythmias).
Concomitant use with drugs that can cause serious interactions leading to toxic effects without proper monitoring or adjustment.
Known severe liver impairment as metabolism may be compromised (clinical judgment required).

Drug Interactions

Inhibitors of cytochrome P450 2D6 (e.g., quinidine, cimetidine, many SSRIs such as fluoxetine, paroxetine, sertraline) increase doxepin plasma levels, increasing risk of toxicity.
Concomitant use with MAO inhibitors is contraindicated due to risk of serious, sometimes fatal, reactions; MAOIs should be discontinued at least 2 weeks prior to doxepin initiation.
Cimetidine therapy can increase serum doxepin levels leading to enhanced anticholinergic side effects; discontinuation of cimetidine may reduce doxepin efficacy.
Alcohol potentiates CNS depressive effects of doxepin increasing sedation and respiratory depression risk.
Tolazamide (antidiabetic agent) co-administration has reported cases of severe hypoglycemia, necessitating close blood glucose monitoring.
Drugs metabolized by CYP2D6 may require dose adjustments when coadministered with doxepin due to competitive metabolism.
Combination with other CNS depressants or sedating drugs may cause additive sedation and respiratory depression.
Concomitant use with drugs causing QT prolongation or cardiac arrhythmias should be avoided to minimize risk of cardiac toxicity.
Certain antiarrhythmic agents (Type 1A and 1C such as quinidine, disopyramide, procainamide) are generally contraindicated during overdose management.
Switching from fluoxetine to doxepin requires a period of at least 5 weeks due to fluoxetine's long half-life to prevent adverse effects.
Concomitant administration with adrenergic antagonists like guanethidine may be inhibited at doses above 150 mg/day.
Interaction with drugs affecting anticholinergic burden can exacerbate side effects such as confusion and urinary retention.
Concurrent use of lithium may increase risk of serotonin syndrome and requires monitoring.
Doxepin may alter the metabolism of other drugs like anticonvulsants, antipsychotics, and beta-blockers via CYP interactions.

Preparations Of The Drug

Doxepin Hydrochloride Oral Solution USP (Concentrate) containing 10 mg doxepin per 1 mL solution.
Inactive ingredients include glycerin, methylparaben, propylparaben, peppermint flavor, water, and may include hydrochloric acid and/or sodium hydroxide for pH adjustment.
Supplied in 120 mL plastic bottles with calibrated droppers for precise dosing (5 mg, 10 mg, 15 mg, 20 mg, and 25 mg measurements).
Instructions advise dilution of each dose with approximately 120 mL (4 ounces) of water or compatible juices (e.g., orange, grapefruit, tomato, prune, pineapple) immediately prior to administration; not compatible with carbonated beverages.
Storage recommendations include maintaining temperature between 20°-25°C (68°-77°F) with permitted excursions between 15°-30°C (59°-86°F) according to USP controlled room temperature guidelines.
Packaging carries label and NDC code 54838-512-40 with marketing by Lannett Company, Inc.
Medication guide accompanies product providing patient counseling information regarding risks, usage, and monitoring.
Prescription-only status with distribution limited to healthcare providers with required supervisory care and patient counseling.
Additional formulations such as capsules and tablets exist but specific details were not provided in this data set.