Concept 02 / 05Chemistry in Everyday Life
Therapeutic Drugs: Antacids, Antihistamines, Tranquillisers and Analgesics
Antacids and H2-receptor antagonists (cimetidine, ranitidine), antihistamines, neurologically active drugs including tranquillisers and antidepressants (iproniazid, phenelzine, chlordiazepoxide, meprobamate, equanil, barbiturates), and non-narcotic (aspirin, paracetamol) and narcotic (morphine, codeine, heroin) analgesics.
Antacids. Excess acid in the stomach causes irritation, pain and, in severe cases, ulcers. Early treatment used weak bases such as sodium hydrogencarbonate or mixtures of aluminium and magnesium hydroxide. Excess makes the stomach alkaline and triggers even more acid production; metal hydroxides are better because they are insoluble and do not raise the pH above neutral. These remedies treat only the symptoms, not the cause.
H2-receptor antagonists. Histamine stimulates the secretion of pepsin and HCl in the stomach. Cimetidine (Tagamet) and later ranitidine (Zantac) block the interaction of histamine with receptors in the stomach wall, reducing acid release. They treat the cause rather than the symptoms.
Antihistamines. Histamine is a potent vasodilator; it contracts smooth muscle in the bronchi and gut and relaxes other muscle such as that in the walls of fine blood vessels. It is responsible for the nasal congestion of colds and for allergic responses to pollen. Brompheniramine (Dimetapp) and terfenadine (Seldane) compete with histamine for the receptor sites where it acts. They do not affect acid secretion, because they act on different receptors from those in the stomach wall.
Neurologically active drugs.
Tranquillisers treat stress and mild or severe mental disease; they relieve anxiety, stress, irritability or excitement, and are an essential component of sleeping pills.
Noradrenaline is a neurotransmitter involved in mood; if its level is low, signalling slows and a person may be depressed. Antidepressants such as iproniazid and phenelzine inhibit the enzymes that break down noradrenaline, so it stays active longer.
Mild tranquillisers: chlordiazepoxide and meprobamate relieve tension. Equanil is used to control depression and hypertension.
Barbiturates (derivatives of barbituric acid, such as veronal, amytal, nembutal, luminal and seconal) are hypnotics (sleep-producing). Valium and serotonin are other substances with tranquillising effects.
Analgesics reduce or abolish pain without impairing consciousness, mental confusion, incoordination or paralysis.
Non-narcotic (non-addictive): aspirin and paracetamol. They also reduce fever (antipyretic). Aspirin inhibits the synthesis of prostaglandins, which cause inflammation and pain. Aspirin also prevents platelet coagulation, so it is used to prevent heart attacks; but it is an acid and can cause stomach bleeding.
Narcotic analgesics: morphine and many of its homologues (codeine, heroin). In medicinal doses they relieve pain and produce sleep; in poisonous doses they cause stupor, coma, convulsions and ultimately death. They are addictive and used mainly for post-operative pain, cardiac pain, pains of terminal cancer and childbirth.
Key Takeaways:
- Metal hydroxide antacids beat (insoluble, no overshoot); cimetidine and ranitidine block acid secretion at its source.
- Antihistamines (brompheniramine, terfenadine) act on allergy receptors, not the stomach.
- Iproniazid and phenelzine raise noradrenaline levels; barbiturates are hypnotics.
- Aspirin and paracetamol are non-narcotic; morphine, codeine and heroin are narcotic.
- Pitfall: aspirin's anti-clotting action is useful for the heart but harmful in ulcers.
Brain-Teaser 1: What mass of neutralises of HCl?
Solution: . , so is needed: .
Brain-Teaser 2: Why does an antihistamine taken for hay fever not reduce stomach acidity, while cimetidine does, though both block histamine?
Solution: Histamine acts on different receptor types. Antihistamines for allergy block the receptors in the airways and blood vessels (H1). Cimetidine blocks the receptors in the stomach wall (H2) that trigger acid secretion. Each drug fits only its receptor type.
Brain-Teaser 3: Why is aspirin, but not paracetamol, prescribed in low doses to prevent heart attacks?
Solution: Aspirin inhibits the synthesis of prostaglandins and thromboxanes involved in platelet aggregation, so it reduces clot formation in blood vessels. Paracetamol has little effect on platelets; it relieves pain and fever but has no significant anti-clotting action.
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