1. Antiemetic drugs

2. 1. Muscarinic blockers Atropine , Hyoscine Antiemetic mechanism:
They block M1 receptors in the vestibulocerebellar pathway, solitary tract nucleus, and CTZ.
Uses as antiemetic :
Prevention (and less effectively to treat) vomiting due to motion sickness
Adverse effects; antimuscarinic side effects

3. 2. H1-blockers Diphenhydramine, Cyclizine, Meclizine
Antiemetic mechanism:
They block H1 (also M1) receptors in the vestibulocerebellar pathway and CTZ.
They have sedative action.
Uses as antiemetic
Vomiting due to motion sickness
Vomiting of pregnancy
Vertigo
Adverse effects:
Sedation (excitation may occur in children).
Atropine-like actions
Hypotension (block alpha1)

4. 3. 5-HT3 blockers Ondansetron, Granisetron, dolasetron
Antiemetic mechanism:
Competitively block 5HT3 receptors in the GIT, solitary tract nucleus and CTZ.
Uses as antiemetic:
Vomiting due to cancer chemotherapy or radiotherapy.
Postoperative nausea and vomiting.
Not effective against motion sickness

5. Adverse effects:
Generally are well tolerated
Dizziness, headache, and constipation.
Prolong QT interval, torsade de points is reported especially if co-administered with another drug that prolong the QT interval

6. 4. Dopamine blockers Benzamides; (Metoclopramide, Domperidone)
Phenothiazines (e.g.  Prochlorperazine, chlorpromazine)
Antiemetic mechanism:
Benzamide
Metoclopramide causes central and peripheral dopamine D2 antagonism at low doses, and weak 5-HT3 blockade 
Domperidone is a D2-blocker with selective peripheral activity in the upper gastrointestinal tract.
Phenothiazines
Antagonizing D2-dopamine receptors in the area postrema of the midbrain, also block M, H1

7. Uses:
Vomiting due to drugs or febrile illness
Vomiting due to cancer chemotherapy.
Postoperative nausea and vomiting.
Adverse effects:
Benzamides;  
Metoclopramide; (anxiety, restlessness, and depression, hyperprolactinemia, irreversible tardive dyskinesia and QT interval prolongation
Domperidone; it does not cross the blood-brain barrier and therefore lacks the neurologic side effects of metoclopramide
Phenothiazine
Sedation, Hyperprolactinemia, Postural hypotension
Extrapyramidal effects e.g. dystonia and dyskinesia.

8. 5. Cannabinoids Dronabinol
A purified synthetic delta-9-tetrahydrocannabinol
Antiemetic mechanism:
It is a cannabinoid receptor agonist.
the drug likely activates specific cannabinoid receptors in the vomiting center, which results in decreased excitability of target neurons
Naloxone blocks its action
Uses as antiemetic:
Vomiting due to cancer chemotherapy
Patients refractory to other antiemetics.

9. Adverse effects:
Sedation
Paranoia
Dysphoria
Hypotension
Drug abuse.
NB;
The modest antiemetic activity of this and their relatively unfavorable side effect profile, especially in older patients, has limited their clinical use.

10. 6. Vitamin B6 Pyridoxine is a water-soluble vitamin
Antiemetic mechanism:
Is unknown
Uses as antiemetic:
Vomiting in pregnancy. Pyridoxine has a good safety profile with minimal side effects 
Vomiting in children

11. 7. Corticosteroids Dexamethasone the commonest steroid used
Antiemetic mechanism;
The exact mechanism is unclear.
Uses as antiemetic:
Effective and well-tolerated antiemetic for chemotherapy induced emesis
Adverse effects:
Hyperglycemia
Immunosuppression
Osteoporosis
Salt and water retention

12. 8. Benzodiazepines
The most commonly used drugs in this class include Lorazepam, alprazolam
As single agents, the benzodiazepines are relatively weak antiemetic agents. They are given primarily as adjunctive agents
Antiemetic mechanism:
Unknown

13. Uses as antiemetic:
Stress-related vomiting
Adjunctive agents to reduce akathisia associated with metoclopramide
To controls symptoms in Ménière disease
Adverse effects:
Anterograde amnesia
Physical dependence
Paradoxical excitement

14. 9. Neurokinin-1 receptor blockers
Aprepitant
Antiemetic mechanism
Neurokinin 1 (NK1) receptor antagonist on nucleus of tractus solitaries and Visceral afferent nerves
Neurokinin-1 receptors mediate most of central and peripheral effects of substance P.
So it prevent both peripheral and central stimulation of the vomiting center

15. Uses as antiemetic:
In combination with 5-HT3 blockers to treat vomiting due to cancer chemotherapy
Adverse effects:
Diarrhea and fatigue
Inhibit CYP3A4 so increase the level of many drugs

16. Smooth Muscle Relaxants
Antispasmodic Drugs

17. Classification Anticholinergic drugs: Atropine Hyoscine
Direct smooth muscle relaxants:
Papaverine
Mebeverine
Alverine, drotaverine.
Combinations:
Libra x ( clidinium + chlordiazepoxide ),
Donnatal (hyoscine + phenobarbital).

18. Direct smooth muscle relaxants
Papaverine:It is opium alkaloid but chemically different from morphine
Mebeverine, Alverine, Drotaverine They are synthetic drugs
Mechanism of action:
The exact mechanism is unclear but may be due to inhibition of PDE enzyme increase cAMP . Smooth muscle relaxation.
Uses :
Spasms of the GIT, bile duct and genitourinary tract.
Irritable bowel syndrome (IBS).

19. Side effects:
Cardiac arrhythmia.
Abnormal liver functions in the form of ↑ serum transaminases and alkaline phosphatase.
Headache and dizziness
C/I :
Paralytic ileus.
Constipation for more than one week