Laxatives and Antidiarrheal drugs

Question 1

Classification.

Answer 1

1. Irritants and stimulants (Senna, Bisacodyl and Castor oil).
2. Bulk Laxatives (Hydrophillic Colloids)
3. Saline and Osmotic Laxatives (Saline cathartics - Magnesium citrate and magnesium hydroxide) + “Lactulose” - Lactic, formic and acetic acid.
4. Stool softners ( Docusate sodium and Docusate calcium )
5. Lubricant laxatives (Mineral oil)
6. Chloride channel activators ( Lubiprostone ).

Question 2

Pharmacodynamics of these drugs?

Answer 2

1. Irritants and stimulants:
   - SENNA : Excretion of water and electrolytes into the bowel, facilitating evacuation of stool, increased intestinal motility. Can treat opioid induced constipations.
   - BISACODYL : Stimulates nerve fibers of the intestinal mucosa, stimulating peristalsis.
   - Castor oil : Degraded to for “RICINOLEIC ACID” which stimulates peristalsis. Contraindicated in pregnancy as it may stimulate uterine contractions.
2. Bulk Laxatives:
   - HYDROPHILLIC COLLOIDS : Form gels in the large intestine that attract water and lead to intestinal distention, thus stimulating peristalsis.
3. Saline and Osmotic Laxatives:
   - SALINE CATHARTICS : Magnesium Citrate and Magnesium Hydroxide cause retention of water in the bowel, causing distention, stimulating defecation.
   - LACTULOSE : Degraded by the colonic bacteria into lactic, formic and acetic acid (acts as osmotic laxatives) by causing the accumulation of water in the bowel, distention which forms soft stools and defecation. Can be used for the treatment of hepatic encephalopathy as it can reduce the concentration of non metabolised ammonia.
4. Stool softeners:
   - DOCUSATE SODIUM/DOCUSATE CALCIUM : Function to soft stool, allowing for easy passage through the bowel.
5. Stool lubricant:
   - MINERAL OIL : Facilitate the passage of hard stool through the bowel.
6. Chloride channel activators:
   - LUBIPROSTONE : Activation of chloride channels leads to increased secretions in the bowel, producing increased peristalsis and soft stools.

Question 3

Adverse effects of Laxatives?

Answer 3

Chronic use of laxatives can lead to electrolyte imbalances due to excessive secretions from bowel into stool. Drugs with delayed onset of action, poor oral absorption have a lower bioavailability due to the increased peristaltic action of the bowel.

Question 4

Classification of Anti-diarrheal drugs?

Answer 4

1. Anti-motility agents : LOPERAMIDE and DIPHENOXYLATE.
2. Adsorbants : ALUMINUM HYDROXIDE and METHYLCELLULOSE.
3. Electrolyte transport modifiers : BISMUTH SUBSALICYLATE.

Question 5

Pharmacodynamics of the Anti-diarrheal drugs?

Answer 5

1. Anti-peristalsis agents : Decrease peristalsis via activation of presynaptic opioid receptors of “enteric nervous system”, inhibiting the release of ACh thus reducing peristalsis.
2. Adsorbants : Function to develop protection of the intestinal mucosa, absorbance of intestinal bacteria/toxins causing diarrhoea.
3. Electrolyte transport modifiers : Reduce the amount of secretions released into the bowel.