Antacids, Antihistamines — Revision Notes

⚡ 30-Second Revision

Antacids: — Weak bases, neutralize stomach HCl. Examples: $Mg(OH)_2$, $Al(OH)_3$, $CaCO_3$, $NaHCO_3$.
Mechanism: — Acid-base neutralization.
Side effects: — $Mg(OH)_2$ (diarrhea), $Al(OH)_3$ (constipation), $CaCO_3$ ($CO_2$ gas, acid rebound, constipation), $NaHCO_3$ ($CO_2$ gas, high Na+).
Antihistamines: — Block histamine receptors.
H1-Antagonists (Allergies):

- First-gen: Sedating (cross BBB). E.g., Diphenhydramine, Chlorpheniramine. - Second-gen: Non-sedating (don't cross BBB). E.g., Loratadine, Cetirizine, Fexofenadine.

H2-Antagonists (Gastric Acid): — Reduce acid secretion. E.g., Ranitidine, Cimetidine, Famotidine.

2-Minute Revision

Antacids are weak bases that provide quick relief from heartburn and indigestion by directly neutralizing excess stomach acid. Common examples include magnesium hydroxide, aluminum hydroxide, and calcium carbonate. Remember that magnesium salts can cause diarrhea, aluminum and calcium salts can cause constipation, and carbonates/bicarbonates produce $CO_2$ gas, leading to belching. They offer symptomatic relief but don't stop acid production.

Antihistamines, on the other hand, block the action of histamine, a chemical released during allergic reactions or to stimulate stomach acid. For allergies, H1-receptor antagonists are used. These are divided into first-generation (e.

g., diphenhydramine), which cause drowsiness because they cross the blood-brain barrier, and second-generation (e.g., loratadine), which are non-sedating as they do not readily cross the BBB. A different class, H2-receptor antagonists (e.

g., ranitidine), reduce stomach acid production by blocking H2 receptors, offering a more sustained effect than antacids. Key for NEET is to distinguish between these classes based on their mechanism, examples, and side effects.

5-Minute Revision

Let's consolidate our understanding of Antacids and Antihistamines, crucial for NEET. Antacids are your first line of defense against hyperacidity. They are simple inorganic bases like magnesium hydroxide ($Mg(OH)_2$), aluminum hydroxide ($Al(OH)_3$), calcium carbonate ($CaCO_3$), and sodium bicarbonate ($NaHCO_3$).

Their mechanism is straightforward: they react with the stomach's hydrochloric acid (HCl) to form a salt and water, thereby increasing the pH and reducing irritation. For example, $Mg(OH)_2 + 2HCl \rightarrow MgCl_2 + 2H_2O$.

Remember the specific side effects: magnesium causes diarrhea, aluminum and calcium cause constipation. Calcium carbonate and sodium bicarbonate also produce $CO_2$ gas, leading to belching. Antacids offer rapid but temporary relief and do not prevent acid production.

Now, for antihistamines, the picture is more nuanced as they target specific receptors. Histamine is a key mediator in allergies and gastric acid secretion. Antihistamines work as receptor antagonists, blocking histamine's binding to its receptors. We primarily focus on two types:

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H1-receptor antagonists: — These are for allergic reactions. They alleviate symptoms like sneezing, itching, and runny nose. They are further categorized:

* First-generation: E.g., Diphenhydramine, Chlorpheniramine. These are lipophilic and cross the blood-brain barrier (BBB), causing sedation and anticholinergic effects. Useful for acute allergies, motion sickness, and as sleep aids. * Second-generation: E.g., Loratadine, Cetirizine, Fexofenadine. These are designed to be less lipophilic and do not readily cross the BBB, making them non-sedating. Preferred for chronic allergy management.

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H2-receptor antagonists: — E.g., Ranitidine, Cimetidine, Famotidine. These block H2 receptors on parietal cells in the stomach, reducing gastric acid secretion. Unlike antacids, they prevent the production of new acid, offering a more sustained effect for conditions like GERD and peptic ulcers. They are distinct from antacids in their mechanism.

Key takeaway for NEET: Understand the specific mechanism (neutralization vs. receptor blockade), classify drugs into their respective categories, and associate common examples with their characteristic side effects (e.g., sedation for first-gen H1, bowel issues for antacids). Differentiating between antacids and H2-blockers is a common question type.

Prelims Revision Notes

Antacids

Definition: — Weak bases that neutralize excess stomach acid.
Mechanism: — Direct acid-base neutralization reaction with HCl.

* $Mg(OH)_2(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + 2H_2O(l)$ * $Al(OH)_3(s) + 3HCl(aq) \rightarrow AlCl_3(aq) + 3H_2O(l)$ * $CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$ * $NaHCO_3(s) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l) + CO_2(g)$

Common Components & Side Effects:

* Magnesium hydroxide ($Mg(OH)_2$): Laxative effect (diarrhea). * Aluminum hydroxide ($Al(OH)_3$): Constipation. * Calcium carbonate ($CaCO_3$): Constipation, acid rebound, $CO_2$ production (belching). * Sodium bicarbonate ($NaHCO_3$): $CO_2$ production (belching), high sodium content (caution for hypertension).

Onset/Duration: — Rapid onset, short duration.
Use: — Symptomatic relief for occasional heartburn, indigestion.

Antihistamines

Definition: — Drugs that block the action of histamine by acting as receptor antagonists.
Histamine: — Biogenic amine, involved in allergies, inflammation, gastric acid secretion.
Types of Receptors: — H1, H2, H3, H4.

H1-Receptor Antagonists (for Allergies)

Mechanism: — Block histamine at H1 receptors, reducing allergic symptoms.
First-Generation:

* Examples: Diphenhydramine, Chlorpheniramine, Promethazine. * Property: Lipophilic, cross blood-brain barrier (BBB). * Side Effect: Sedation/drowsiness, anticholinergic effects (dry mouth, blurred vision). * Uses: Acute allergies, motion sickness, insomnia.

Second-Generation:

* Examples: Loratadine, Cetirizine, Fexofenadine, Desloratadine. * Property: Less lipophilic, do not readily cross BBB (or actively pumped out). * Side Effect: Non-sedating, fewer anticholinergic effects. * Uses: Chronic allergy management.

H2-Receptor Antagonists (for Gastric Acid Reduction)

Mechanism: — Block histamine at H2 receptors on parietal cells in the stomach, reducing gastric acid secretion.
Examples: — Ranitidine, Cimetidine, Famotidine, Nizatidine.
Onset/Duration: — Slower onset than antacids, longer duration.
Use: — Peptic ulcers, GERD, conditions with excessive acid production.
Key Distinction: — Unlike antacids, they prevent acid production, rather than neutralizing existing acid.

Vyyuha Quick Recall

For Antihistamines: H1 for Hives (allergies), H2 for Heartburn (acid).

For Sedating vs. Non-sedating H1 blockers: First-gen Falls asleep (sedating), Second-gen Stays awake (non-sedating).