Pharmacology of laxatives and emetics Flashcards
What are laxatives used for and list the types?
laxatives used for constipation - which is caused by excess water reabsorption and inadequate peristalsis usually in patients with fibre deficient diets
bulk forming agents
osmotic laxatives
stimulant laxatives
lubricant laxatives
secretagogues
role of Bulk forming agents?
e.g. Isphalga hulk, methylcellulose, bran
attract water, swell in colon which promotes peristalsis
take several days to start working
role of stimulant laxatives?
e.g. senna, bisocadyl
usually taken if osmotic doesn’t work
stimulate enteric nerves to cause peristalsis, works in around 8 hrs but can cause cramps
role of lubricant laxatives?
e.g. docusate sodium
stool softeners, useful in management of haemorrhoids and anal fissures
role of osmotic laxatives?
e.g. lactulose, macrogol
stimulate fluid movement into intestinal lumen by osmosis
take 24/48hrs to work
role of secretagogues?
new agents - used in chronic constipation when conventional laxatives are unresponsive
prucalopride - 5HT4 agonist, increases colonic transit by activating 5HT4 receptor in colonic mucosa
lubiprostone - chloride channel activator - enhances mucosal Cl- and fluid secretion - increases colonic motility
Treatments in diarrhoea?
ORT - aims to replace lost electrolytes and water to prevent dehydration and electrolyte imbalance
narcotic anti-diarrhoeal - loperamide, codeine phosphate - reduce intestinal hypermotility
ciprofloxacin - prophylactic antibiotic - for travellers diarrhoea
charcoal
kaolin
Treatments in IBS?
irritable bowel syndrome
- alternating constipation + diarrhoea
- recurrent abdominal pain, bloating
- change in bowel habit
constipation - bulk forming agent
anti-diarrhoeal
anti-spasmodic - mebeverine for abdominal pain
peppermint oil - relaxant on intestinal smooth muscle
anti-flatulent for flatulence and bloating
don’t give bulk forming agent and mebeverine together as can cause intestinal obstruction
what are the benefits of dietary fibre?
indigestible fibre (cellulose from plant cell wall)
- gives bulk - increases mechanical efficiency of intestine
- speeds up transit
- adsorbs water - results in soft stools
- promotes regular bowel habit
How do PNS and CNS mechanisms result in vomiting reflex?
nausea & vomiting - common nonspecific features of disease and drug toxicity
- activation of chemoreceptor trigger zone (CTZ), via vagal nerves in stomach and involvement of vestibular apparatus
when CTZ is activated - sends signals to VC - induces vomiting response
- CTZ can be directly activated from cytotoxic drugs in chemotherapy
- or irritants in stomach can activate vagal sensory fibres which then send signals to CTZ and VC in brain
- in motion sickness - vestibular branch of cochlea nerve gets stimulated - sends signals to vestibulo nucleus - fibres projected to CTZ - once CTZ is activated sends signals to VC - vomiting occurs
list drugs that cause emesis?
cytotoxic drugs
list drugs that inhibit emesis?
D2 antagonists - prochlorperazine, domperidone
- D2 receptors in CTZ
5HT3 antagonists - ondansetron
- 5HT3 receptors in CTZ and on cell bodies of vagal sensory nerves
anticholinergics - muscarinic antagonists - hyoscine
- muscarinic receptors in VC
sedating antihistamines - H1 antagonists - promethazine, cinnarizine
- histamine H1 receptors in VC
neurokinin 1 receptor antagonist - aprepitant
- NK1r in VC
dexamethasone - anti-emetic properties esp in vomiting associated with chemotherapy
Treatment for radiation and cytotoxic drug induced emesis?
5HT3 antagonist (ondansetron) - effective anti-emetics esp. for vomiting associated with anti cancer chemotherapy
dexamethasone - anti emetic corticosteroid
if risk of emesis is high - combination therapy
- 5HT3 antagonist + dexamethasone + NK1 antagonist
