Antiemetics

Antiemetics are used to relieve nausea and prevent vomiting.

The mechanism of vomiting is highly complex, as it involves a common reflex response to a wide variety of stimuli and pathophysiological conditions.

The vomiting center in the brain receives input from various sources, including the inner ear, gastrointestinal tract, brain, and even the testicles. These reflexes are triggered through multiple communication pathways, which is why there are diverse medications with varying mechanisms of action to control vomiting.

Since nausea and vomiting are natural defense mechanisms of the body, it is crucial to identify their underlying cause rather than merely treating the symptoms, as doing so could mask a serious illness.

Nausea can serve a protective role by discouraging further consumption of a harmful substance, while vomiting helps expel potential toxins.

In most acute situations, vomiting is temporary and can improve with hydration, either by sipping liquids gradually or through the administration of intravenous fluids.

The use of antiemetic medications is rare and should be approached cautiously, as they can have significant adverse effects.

The most common situations requiring a prescription for antiemetics include:

• Pregnant women experiencing morning sickness or hyperemesis gravidarum.
• Cancer patients undergoing chemotherapy or radiotherapy.

In both cases, it is essential for the doctor to carefully select the appropriate medication to avoid worsening other symptoms or causing undesirable side effects.

There are many medications that can act as antiemetics. The main groups of antiemetic drugs include:

• 5-HT3 receptor antagonists: These drugs block serotonergic HT3 receptors and are among the most potent antiemetics, especially for managing vomiting caused by chemotherapy, radiotherapy, and postoperative conditions.

  They directly inhibit the vomiting reflex at the neurological level. Examples include: ondansetron, granisetron, dolasetron, tropisetron y palonosetron.

  These medications can be administered intramuscularly or intravenously, depending on the specific drug. They are sometimes combined with systemic corticosteroids, such as dexamethasone, to enhance their effectiveness.

  Side effects: Headache, constipation or diarrhea, confusion, and a risk of serotonin syndrome (symptoms include tremors, diarrhea, fever, and seizures). In some cases, they may cause heart problems and are generally avoided during the first 12 weeks of pregnancy due to unclear risks of fetal malformations.

• Dopamine Receptor Antagonists
  • Phenothiazines (e.g., prochlorperazine, chlorpromazine): Useful for motion sickness and nausea/vomiting of gastrointestinal origin. They are available as oral tablets, injections, and suppositories.Side effects: Extrapyramidal symptoms (e.g., dystonia), cardiac effects, and hypotension. Not recommended for elderly patients, especially those with dementia.
  • Benzamides/Orthopramides (e.g., metoclopramide, domperidone, clebopride): Stimulate intestinal motility and are used for nausea and vomiting. Trimethobenzamide is particularly useful after surgery.
    Side effects: Extrapyramidal effects and other neurological symptoms.
  • Olanzapine: Primarily a psychiatric medication, it is effective for chemotherapy-induced nausea and vomiting. However, it has numerous side effects affecting the nervous system and heart.
• Anticholinergic agents: Scopolamine blocks cholinergic and histamine receptors, making it effective for motion sickness originating from the inner ear.
  Side effects: Dry mouth and drowsiness.
• H1 Antihistamines: Drugs like cyclizine, meclizine, promethazine, dimenhydrinate, and diphenhydramine are useful for motion sickness and postoperative vomiting.
  Side effects: Less potent than scopolamine but cause drowsiness.
• Neurokinin-1 (NK-1) receptor antagonists: Drugs like aprepitant, olapitant, and the combination of netupitant with palonosetron are used for chemotherapy-induced nausea and vomiting.
• Cannabinoids: Cannabis-derived products (dronabinol and nabilone) are sometimes used for chemotherapy-related vomiting but are less effective and have more side effects than serotonin antagonists.
• Glucocorticoids and anti-inflammatory agents: Dexamethasone enhances the antiemetic effects of other drugs like ondansetron or metoclopramide. The ondansetron/dexamethasone combination is considered one of the most potent antiemetic therapies available.
• Benzodiazepines: Drugs like diazepam and lorazepam are used as adjuncts to other treatments for chemotherapy-induced nausea.
• Phosphorus carbohydrate solutions: Aqueous solutions of glucose, fructose, and orthophosphoric acid are available for relieving mild nausea.
• Doxylamine and pyridoxine succinate: Commonly used to treat nausea and vomiting during pregnancy.
• Antivertigo agents: Betahistine is primarily used for Ménière’s disease, which causes vertigo, nausea, vomiting, hearing loss, and tinnitus.