Doxepin dosage

There has been a report of an 8-week-old breast-fed infant whose mother was taking doxepin (143). Four days after an increase in the daily dosage from 10 to 75 mg the infant developed respiratory depression desmethyldoxe-pin was detected in the baby s plasma. 

Peak concentrations of these drugs are achieved rapidly in the skin and persist after plasma levels have declined. This is consistent with inhibition of wheal and flare responses to the intra-dermal injection of histamine or allergen, which last for 36 hours or more after treatment, even when concentrations in plasma are very low. Such results emphasize the need for flexibihty in the interpretation of the recommended dosage schedules (Table 24-2) less frequent dosage may suffice. Doxepin, a tricyclic antidepressant (see Chapter 17), is one of the most potent antihistamines available it is -800 times more potent than diphenhydramine. This may account for the observation that topical doxepin can be effective in the treatment of chronic urticaria when other antihistamines have failed. 

A study found that carbamazepine reduced the serum levels of nortriptyi-ine by 58% and of amitriptyline plus its metabolite, nortriptyline, by 60% in 8 psychiatric patients. In 17 other patients carbamazepine reduced serum doxepin levels by 54% and doxepin plus its metabolite, nordox-epin, by 55%. A retrospective study of very large numbers of patients confirmed that carbamazepine approximately halves the serum levels of amitriptyline and nortriptyline. An elderly woman needed her nortriptyline dosage to be inereased from 75 to 150 mg daily to achieve effective antidepressant serum levels when carbamazepine 500 to 600 mg daily was added. ... 

The reduction in the serum levels of amitriptyline, desipramine, doxepin, imipramine and nortriptyline caused by the interaction with carbamazepine appears to be established but the clinical importance is very much less certain. Evidence from one study, that achieved a beneficial response in patients taking tricyclics and carbamazepine suggests that it is possibly not necessary to increase the tricyclic dosage to accommodate this interaction. The fact that a retrospective study found that increased imipramine doses were being given to those taking carbamazepine suggests that this interaction will be naturally accounted for. If carbamazepine is added to treatment with any of these tricyclics, be aware that the dose of the tricyclic may need to be titrated up to achieve the desired therapeutic response. Remember too that the tricyclics can lower the convulsive threshold and should therefore be used with caution in patients with epilepsy. 

The interactions with cimetidine are well established, well documented and of clinical importance. The incidence is uncertain hut most patients could be affected. Those taking amitriptyline, desipramine, doxepin, imi-pramine or nortriptyline who are given cimetidine should be warned that adverse effects such as mouth dryness, urine retention, blurred vision, constipation, tachycardia, postural hypotension may be more likely to occur. Other tricyclic antidepressants would be expected to be similarly affected. If symptoms are troublesome reduce the dosage of the antidepressant (33 to 50% has been suggested) or replace the cimetidine with ranitidine, which does not appear to interact. Other H2-ieceptor antagonists that do not cause enzyme inhibition (e.g. famotidine and nizatidine) would also not be expected to interact. 

Po and Irwin (1979) used TLC to separate numerous tricyclic neuroleptic tranquilizers. Samples were dissolved in ethyl acetate, and the mobile phase consisted of mixtures of different n-alcohols with water. Circular development in a Camag U chamber was used, and spots were detected by fluorescence quenching. The /Jp values of some of the better known drugs developed in methanol-water (90 10) were amitriptyline, 0.17 clopenthixol, 0.40 doxepin, 0.42 nortriptyline, 0.37 and promazine, 0.14. Shirke et al. (1994) determined amitriptyline and chlordiazepoxide in combined dosage forms using ethyl acetate-methanol-dieth-ylamine (9.5 0.5 0.05) mobile phase and scanning at 245 nm. 

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