"Scientists at the National Institutes of Health report they have discovered in mouse studies that a small molecule released in the spinal cord triggers a process that is later experienced in the brain as the sensation of itch.
Controlled Clinical Trials
Systemic Adverse Effects: In controlled clinical trials of patients treated with Zonalon (doxepin) Cream, the most common systemic adverse event reported was drowsiness. Drowsiness occurred in 71 of 330 (22%) of patients treated with Zonalon (doxepin) Cream compared to 7 of 334 (2%) of patients treated with vehicle cream. Drowsiness resulted in the premature discontinuation of the drug in approximately 5% of patients treated with Zonalon (doxepin) Cream in controlled clinical trials.
Local Site Adverse Effects: In controlled clinical trials of patients treated with Zonalon (doxepin) Cream, the most common local site adverse event reported was burning and/or stinging at the site of application. These occurred in 76 of 330 (23%) of patients treated with Zonalon (doxepin) Cream compared to 54 of 334 (16%) of patients treated with vehicle cream. Most of these reactions were categorized as "mild"; however, approximately 25% of patients who reported burning and/or stinging reported the reaction as "severe". Four patients treated with Zonalon (doxepin) Cream withdrew from the study because of the burning and/or stinging.
The table below presents the adverse events reported at an incidence of ≥ 1 % in either Zonalon (doxepin) or vehicle cream treatment groups during the trials:
|Burning /Stinging||76 (23.0%)||54 (16.2%)|
|Drowsiness||71 (21.5%)||7 (2.1%)|
|Dry Mouth1||32 (9.7%)||4 (1.2%)|
|Pruritus2||13 (3.9%)||20 (6.0%)|
|Fatigue/Tiredness||10 (3.0%)||5 (1.5%)|
|Exacerbated Eczema||10 (3.0%)||8 (2.4%)|
|Other Application Site Reaction3||10 (3.0%)||16 (4.8%)|
|Dizziness4||7 (2.1%)||3 (0.9%)|
|Mental Emotional Changes||6 (1.8%)||1 (0.3%)|
|Taste Perversion5||5 (1.5%)||1 (0.3%)|
|Edema||4 (1.2%)||1 (0.3%)|
|Headache||3 (0.9%)||14 (4.2%)|
|1 Includes reports of "dry lips", "dry
throat", and "thirst"
2Includes reports of "Pruritus Exacerbated"
3Includes report of "increased irritation at application site"
4Includes reports of "lightheadedness" and "dizziness/vertigo"
5Includes reports of "bitter taste" and "metallic taste in mouth"
Post Marketing Experience
Twenty-six cases of allergic contact dermatitis have been reported in patients using Zonalon (doxepin) Cream, twenty of which were documented by positive patch test to doxepin 5% cream.
Read the Zonalon (doxepin) Side Effects Center for a complete guide to possible side effects
Studies have not been performed examining drug interactions with Zonalon (doxepin) Cream. However, since plasma levels of doxepin following topical application of Zonalon Cream can reach levels obtained with oral doxepin HCl therapy, the following drug interactions are possible following topical Zonalon (doxepin) Cream application:
Drugs Metabolized by P450 2D6
The biochemical activity of the drug metabolizing isozyme cytochrome P450 2D6 (debrisoquin hydroxylase) is reduced in a subset of the Caucasian population (about 7-10% of Caucasians are so-called "poor metabolizers"); reliable estimates of the prevalence of reduced P450 2D6 isozyme activity among Asian, African and other populations are not yet available. Poor metabolizers have higher than expected plasma concentrations of tricyclic antidepressants (TCAs) when given usual doses. Depending on the fraction of drug metabolized by P450 2D6, the increase in plasma concentration may be small, or quite large (8-fold increase in plasma AUC of the TCA).
In addition, certain drugs inhibit the activity of this isozyme and make normal metabolizers resemble poor metabolizers. An individual who is stable on a given dosage regimen of a TCA may become abruptly toxic when given one of these inhibiting drugs as concomitant therapy. The drugs that inhibit cytochrome P450 2D6 include some that are not metabolized by the enzyme (quinidine; cimetidine) and many that are substrates for P450 2D6 (many other antidepressants, phenothiazines, and the Type 1C antiarrhythmics propafenone and flecainide). While all the selective serotonin reuptake inhibitors (SSRIs), e.g., fluoxetine, sertraline, and paroxetine, inhibit P450 2D6, they may vary in the extent of inhibition. The extent to which SSRI-TCA interactions may pose clinical problems will depend on the degree of inhibition and the pharmacokinetics of the SSRI involved. Nevertheless, caution is indicated in the co-administration of TCAs with any of the SSRIs. Of particular importance, sufficient time must elapse before initiating TCA treatment in a patient being withdrawn from fluoxetine, given the long half-life of the parent and active metabolite (at least 5 weeks may be necessary).
Concomitant use of tricyclic antidepressants with drugs that can inhibit cytochrome P450 2D6 may require lower doses than usually prescribed for either the tricyclic antidepressant or the other drug. It is desirable to monitor TCA plasma levels whenever a TCA is going to be co-administered with another drug known to be an inhibitor of P450 2D6.
MAO Inhibitors: Serious side effects and even death have been reported following the concomitant use of certain drugs with MAO inhibitors. Therefore, MAO inhibitors should be discontinued at least two weeks prior to the cautious initiation of therapy with Zonalon (doxepin) Cream. The exact length of time may vary and is dependent upon the particular MAO inhibitor being used, the length of time it has been administered, and the dosage involved.
Cimetidine: Serious anticholinergic symptoms (i.e., severe dry mouth, urinary retention and blurred vision) have been associated with elevations in the serum levels of tricyclic antidepressant when cimetidine therapy is initiated. Additionally, higher than expected tricyclic antidepressant levels have been observed when they are begun in patients already taking cimetidine.
Tolazamide: A case of severe hypoglycemia has been reported in a type II diabetic patient maintained on tolazamide (1 gm/day) 11 days after the addition of oral doxepin (75 mg/day).
Read the Zonalon Drug Interactions Center for a complete guide to possible interactions
Last reviewed on RxList: 12/29/2008
This monograph has been modified to include the generic and brand name in many instances.
Additional Zonalon Information
- Zonalon Drug Interactions Center: doxepin top
- Zonalon Side Effects Center
- Zonalon Overview including Precautions
- Zonalon FDA Approved Prescribing Information including Dosage
Zonalon - User Reviews
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