phenothiazine
Meaning of phenothiazine: The three-ring chemical skeleton that gave psychiatry its first effective antipsychotics and medicine several reliable anti-sickness drugs.
Definition of phenothiazine
Phenothiazine is a three-ring chemical structure that forms the backbone of a family of medicines, the best known being the first-generation antipsychotics. Drugs built on it block dopamine receptors in the brain, which eases hallucinations and fixed false beliefs. Several members also block the signals that trigger vomiting. Chlorpromazine and prochlorperazine are familiar examples.
What are phenothiazines used for?
One chemical family covers two very different jobs. Antipsychotic treatment uses much larger amounts and continues for months or years. Anti-sickness use relies on smaller amounts for a short period, which is why the same drug can appear on one chart as a psychiatric medicine and on another as an anti-emetic.
| Use | Example | What the drug does |
|---|---|---|
| Psychosis | Chlorpromazine, fluphenazine | Blocks dopamine signaling in the brain |
| Severe sickness | Prochlorperazine, promethazine | Damps the vomiting center in the brainstem |
| Vertigo | Prochlorperazine | Reduces signals from the balance pathways |
| Persistent hiccups | Chlorpromazine | Settles the reflex that drives them |
How do they work?
Dopamine is a chemical messenger nerve cells use to signal one another. A phenothiazine sits on the dopamine receptor and stops the message being received. Blocking dopamine in one brain pathway reduces the hallucinations and delusions of psychosis. Blocking it in a second pathway, the one governing movement, produces stiffness, tremor and restlessness. That second effect comes with the first, and it is the reason these drugs are called typical antipsychotics. Most phenothiazines also block histamine, acetylcholine and adrenaline receptors, which explains drowsiness, dry mouth, blurred vision and lightheadedness on standing up.
How do they differ from antidepressants?
They act on different messengers and treat different illnesses. An antidepressant such as fluoxetine raises serotonin activity and is prescribed for depression and anxiety. A phenothiazine blocks dopamine and is prescribed for psychosis or for severe nausea. The two classes are sometimes given together, but neither substitutes for the other.
What are the serious risks?
Long use can cause tardive dyskinesia, involuntary movements of the face, tongue and limbs that may persist after the drug is stopped. Neuroleptic malignant syndrome is rare and dangerous, bringing high fever, muscle rigidity, confusion and unstable blood pressure. Some members lengthen the heart's electrical recovery time. Falls in white cell counts and sensitivity of the skin to sunlight are also recognized.
Used in a sentence
The chart showed a phenothiazine prescribed for intractable nausea rather than for a psychiatric indication.
Good to know
Because phenothiazines block dopamine, movement side effects are an expected consequence rather than an unusual reaction, and tardive dyskinesia can persist after the drug is withdrawn.