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Phenothiazine – Effects, Uses & Side Effects

Phenothiazine is a chemical scaffold that forms the basis of an important class of drugs used primarily as antipsychotics and antiemetics in clinical medicine.

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Things worth knowing about "Phenothiazine"

Phenothiazine is a chemical scaffold that forms the basis of an important class of drugs used primarily as antipsychotics and antiemetics in clinical medicine.

What is Phenothiazine?

Phenothiazine is a heterocyclic organic compound that serves as the core structure for a large and well-studied class of pharmacologically active substances. Known as phenothiazine derivatives, these drugs include well-known agents such as chlorpromazine, promethazine, thioridazine, and trifluoperazine. Phenothiazine derivatives are primarily used as antipsychotics (neuroleptics), antiemetics (for nausea and vomiting), and antihistamines. Chlorpromazine, introduced in 1952, was the first antipsychotic drug ever used in clinical psychiatry and marked a revolution in the treatment of schizophrenia.

Mechanism of Action

Phenothiazine derivatives exert their pharmacological effects primarily through the blockade of dopamine receptors, especially the D2 subtype, in the central nervous system. This dopamine-blocking effect underlies their antipsychotic properties. Many phenothiazines also interact with other receptor systems:

Histamine H1 receptors: Blockade produces sedative and antiallergic effects
Muscarinic acetylcholine receptors: Blockade causes anticholinergic effects (e.g., dry mouth, urinary retention)
Alpha-adrenergic receptors: Blockade can result in lowered blood pressure (hypotension)

The antiemetic effect is due to dopamine receptor blockade in the chemoreceptor trigger zone (CTZ) of the brainstem, which plays a central role in initiating nausea and vomiting.

Medical Applications

Antipsychotic Therapy

Phenothiazines such as chlorpromazine and perazine are used to treat schizophrenia and other psychotic disorders. They help reduce symptoms including hallucinations, delusions, and disorganized thinking. These agents belong to the class of typical antipsychotics (also referred to as first-generation antipsychotics or conventional neuroleptics).

Antiemetic Therapy

Agents such as promethazine and prochlorperazine are used to control nausea and vomiting, including postoperative nausea, chemotherapy-induced emesis, and motion sickness.

Sedation and Anxiolysis

Some phenothiazines have strong sedative properties and are used to manage severe agitation or anxiety, as well as for preoperative sedation in clinical settings.

Antihistamine Use

Promethazine is also widely used as an antihistamine for allergic reactions and as a short-term treatment for insomnia due to its sedating properties.

Dosage and Administration

The dosage of phenothiazine derivatives varies greatly depending on the specific agent and indication, and must be individually tailored to each patient based on age, body weight, and comorbidities. Administration can be oral (tablets or liquid), intramuscular, or intravenous depending on the clinical situation. All phenothiazine derivatives are prescription-only medications and must be used under medical supervision.

Side Effects

Phenothiazines can cause a range of side effects, some of which are dose-dependent:

Extrapyramidal symptoms (EPS): Muscle stiffness, tremor, involuntary movements (tardive dyskinesia), akathisia (inner restlessness)
Sedation: Drowsiness and fatigue, especially at the beginning of therapy
Anticholinergic effects: Dry mouth, constipation, urinary retention, blurred vision
Orthostatic hypotension: Drop in blood pressure upon standing
Weight gain and metabolic effects
Elevated prolactin levels (hyperprolactinemia), which may lead to menstrual irregularities or galactorrhea
Neuroleptic malignant syndrome (NMS): A rare but life-threatening complication characterized by high fever, severe muscle rigidity, and altered consciousness
QT prolongation: Changes in cardiac conduction that can increase the risk of dangerous arrhythmias

Contraindications and Drug Interactions

Phenothiazines are contraindicated in patients with:

Known hypersensitivity to phenothiazines
Severe hepatic impairment
Bone marrow depression
Narrow-angle glaucoma
Concurrent use of other QT-prolonging substances

Important interactions exist with alcohol, other central nervous system depressants, antihypertensive drugs, and other QT-prolonging medications. Use during pregnancy and breastfeeding requires careful medical evaluation and risk-benefit assessment.

References

Stahl, S. M. - Stahl's Essential Psychopharmacology: Neuroscientific Basis and Practical Applications. 4th Edition. Cambridge University Press, 2013.
Brunton, L. L. et al. - Goodman & Gilman's: The Pharmacological Basis of Therapeutics. 13th Edition. McGraw-Hill, 2018.
Leucht, S. et al. - Comparative efficacy and tolerability of 15 antipsychotic drugs in schizophrenia: a multiple-treatments meta-analysis. The Lancet, 382(9896), 951-962, 2013.

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