Pharmacology · Short notes

Respiratory and GI Drugs: Short Notes & Cheat Sheet

Asthma and COPD drugs, acid suppression, H. pylori regimens, antiemetics, laxatives, antidiarrhoeals and IBD drugs.

Short notes

Read these before you attempt the topic's MCQs.

How acid-suppressing drugs work on the parietal cell

H2 blockers remove one stimulus, but PPIs block the final common pathway (the proton pump itself), which is why they suppress acid more completely.

Asthma and COPD

Short-acting β2 agonists (SABA, e.g. salbutamol) relieve symptoms. Side effects: tremor, tachycardia, hypokalaemia.
Long-acting β2 agonists (LABA: salmeterol, formoterol) must never be used alone in asthma. Always combine them with an inhaled corticosteroid (ICS).
Inhaled corticosteroids (beclomethasone, budesonide, fluticasone) are the core controller therapy. They cause oral thrush and hoarseness, so patients should rinse their mouth after use.
Tiotropium, a long-acting antimuscarinic (LAMA), is a mainstay in COPD. Ipratropium is short-acting.
Theophylline has a narrow therapeutic range and toxicity causes seizures and arrhythmias. Ciprofloxacin and erythromycin raise its level; smoking lowers it.
Montelukast blocks leukotriene receptors and is useful in aspirin-sensitive and exercise-induced asthma. Cromolyn is used only for prevention. Omalizumab targets IgE.
Acute severe asthma: oxygen, nebulised salbutamol with ipratropium, systemic steroids, and IV magnesium sulfate.

Acid-related disease

Proton pump inhibitors (PPIs) are prodrugs that are activated in acid and irreversibly block H+/K+-ATPase. Take them 30–60 minutes before breakfast.
Long-term PPI use can cause low magnesium, B12 deficiency, fractures and C. difficile infection. Omeprazole reduces clopidogrel activation.
H2 blockers: cimetidine inhibits CYP enzymes and causes gynaecomastia. Famotidine is the most potent.
Antacids: magnesium causes diarrhoea and aluminium causes constipation. Both chelate tetracyclines and quinolones.
Sucralfate needs acid to work and coats the ulcer base. Misoprostol (a PGE1 analogue) prevents NSAID ulcers but is contraindicated in pregnancy.
H. pylori: triple therapy is a PPI + clarithromycin + amoxicillin or metronidazole for 14 days. Bismuth quadruple therapy is a PPI + bismuth + tetracycline + metronidazole.

Antiemetics

Ondansetron (5-HT3 antagonist) is used for chemotherapy and post-operative nausea. It can prolong QT and cause constipation.
Metoclopramide (D2 antagonist and 5-HT4 agonist) also speeds gut emptying. It can cause EPS and is contraindicated in GI obstruction.
Domperidone does not cross the blood–brain barrier, so it causes little EPS, but it can prolong QT.
Aprepitant (NK1 antagonist) is used for delayed chemotherapy nausea. Dexamethasone boosts other antiemetics.
Hyoscine and antihistamines such as cyclizine treat motion sickness. Doxylamine with pyridoxine treats nausea in pregnancy.

Bowel drugs

Bulk laxatives (ispaghula) need plenty of water. Osmotic laxatives include lactulose, polyethylene glycol (PEG) and magnesium salts. Stimulant laxatives are bisacodyl and senna. Docusate is a stool softener.
Lactulose also treats hepatic encephalopathy by reducing ammonia absorption.
Long-term liquid paraffin reduces absorption of fat-soluble vitamins. Methylnaltrexone treats opioid-induced constipation.
Diarrhoea: low-osmolarity ORS (245 mOsm/L), plus zinc for children for 10–14 days. Loperamide can be added. Racecadotril is an enkephalinase inhibitor.
IBD: sulfasalazine is 5-ASA linked to sulfapyridine and split by colonic bacteria; it can cause oligospermia and folate deficiency. Mesalazine is 5-ASA alone. Azathioprine requires a TPMT check. Before an anti-TNF drug such as infliximab, screen for latent TB.
Ursodeoxycholic acid is used for gallstones and primary biliary cholangitis. Orlistat inhibits lipase and causes steatorrhoea.

Cheat sheet

High-yield facts in one place. Cover the right-hand column and test yourself.

Mechanisms

Drug	Mechanism
Salbutamol, formoterol	β2 agonist
Ipratropium, tiotropium	Muscarinic antagonist
Theophylline	PDE inhibition, adenosine antagonism
Montelukast	CysLT1 receptor antagonist
Zileuton	5-lipoxygenase inhibitor
Omeprazole	Irreversible H+/K+-ATPase inhibitor
Famotidine	H2 antagonist
Misoprostol	PGE1 analogue
Ondansetron	5-HT3 antagonist
Metoclopramide	D2 antagonist, 5-HT4 agonist
Aprepitant	NK1 antagonist
Lactulose	Osmotic; ↓ NH3 absorption

Pairs examiners love

Question	Answer
ICS side effect	Oral candidiasis (rinse mouth)
Theophylline raised by	Ciprofloxacin, erythromycin, cimetidine
Theophylline lowered by	Smoking, rifampicin, phenytoin
Antacid with diarrhoea	Magnesium hydroxide
Antacid with constipation	Aluminium hydroxide
Black stool without bleeding	Bismuth, iron
Antiemetic without EPS	Domperidone, ondansetron
Hepatic encephalopathy	Lactulose + rifaximin

Memory aids

Magnesium moves, aluminium stops: Mg-containing antacids cause diarrhoea; Al-containing ones cause constipation.
LABA never alone: In asthma a LABA is always given with an inhaled corticosteroid.

Exam traps

PPIs work best when taken before a meal, because they only block pumps that are actively secreting.
Sucralfate needs an acidic pH, so do not give it together with antacids.
Montelukast is a receptor antagonist; zileuton inhibits leukotriene synthesis.
Ranitidine was withdrawn over NDMA contamination, not because of its pharmacology.
Cromolyn is purely preventive and does nothing during an acute attack.

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