Cimetidine

CIMETIDINE- cimetidine tablet, film coated
PD-Rx Pharmaceuticals, Inc.

Cimetidine is a histamine H 2 -receptor antagonist. Chemically it is N” -cyano- N -methyl- N’ -[2-[[(5-methyl-1 H -imidazol-4-yl)methyl]thio]-ethyl]guanidine. Its structural formula is:

Structural Formula

Cimetidine contains an imidazole ring, and is chemically related to histamine.

Cimetidine has a bitter taste and characteristic odor.

Solubility Characteristics

Cimetidine is soluble in alcohol, slightly soluble in water, very slightly soluble in chloroform and insoluble in ether.

Each tablet, for oral administration, contains 200 mg, 300 mg, 400 mg or 800 mg cimetidine, USP. Inactive ingredients are: croscarmellose sodium, crospovidone, hypromellose, lecithin, magnesium stearate, microcrystalline cellulose, polydextrose, polyethylene glycol, povidone, pregelatinized starch (corn), sodium alginate, sodium lauryl sulfate, titanium dioxide, triacetin, vanillin, FD&C Blue No. 1 Aluminum Lake, FD&C Yellow No. 6 Aluminum Lake and D&C Yellow No. 10 Aluminum Lake.

CLINICAL PHARMACOLOGY

Cimetidine tablets competitively inhibits the action of histamine at the histamine H 2 receptors of the parietal cells and thus is a histamine H 2 -receptor antagonist.

Cimetidine is not an anticholinergic agent. Studies have shown that cimetidine tablets inhibit both daytime and nocturnal basal gastric acid secretion. Cimetidine tablets also inhibit gastric acid secretion stimulated by food, histamine, pentagastrin, caffeine and insulin.

Antisecretory Activity

1) Acid Secretion

Nocturnal

An 800 mg oral dose of cimetidine tablets at bedtime reduces mean hourly H + activity by greater than 85% over an 8-hour period in duodenal ulcer patients, with no effect on daytime acid secretion. A 1,600 mg oral dose of cimetidine tablets at bedtime produces 100% inhibition of mean hourly H + activity over an 8-hour period in duodenal ulcer patients, but also reduces H + activity by 35% for an additional 5 hours into the following morning. Cimetidine tablets given as 400 mg twice daily and 300 mg 4 times daily decreases nocturnal acid secretion in a dose-related manner, i.e., 47% to 83% over a 6- to 8-hour period and 54% over a 9-hour period, respectively.

Food Stimulated

During the first hour after a standard experimental meal, a 300 mg oral dose of cimetidine tablets inhibited gastric acid secretion in duodenal ulcer patients by at least 50%. During the subsequent 2 hours cimetidine tablets inhibited gastric acid secretion by at least 75%.

The effect of a 300 mg breakfast dose of cimetidine tablets continued for at least 4 hours and there was partial suppression of the rise in gastric acid secretion following the luncheon meal in duodenal ulcer patients. This suppression of gastric acid output was enhanced and could be maintained by another 300 mg dose of cimetidine tablets given with lunch.

In another study, a 300 mg dose of cimetidine tablets given with the meal increased gastric pH as compared with placebo.

Table 1. Mean Gastric pH

Cimetidine

Placebo

1 hour

3.5

2.6

2 hours

3.1

1.6

3 hours

3.8

1.9

4 hours

6.1

2.2

24-Hour Mean H+ Activity

Cimetidine tablets dosed at 800 mg at bedtime, 400 mg twice daily, and 300 mg 4 times daily, all provide a similar, moderate (less than 60%) level of 24-hour acid suppression. However, the 800 mg bedtime dose regimen exerts its entire effect on nocturnal acid, and does not affect daytime gastric physiology.

Chemically Stimulated

Cimetidine tablets administered orally significantly inhibited gastric acid secretion stimulated by betazole (an isomer of histamine), pentagastrin, caffeine and insulin as follows:

Table 2. Cimetidine Tablets Inhibition of Stimulated Gastric Acid Secretion

Stimulant

Stimulant Dose

Cimetidine Tablets

% Inhibition

Betazole

1.5 mg/kg (sc)

300 mg (po)

85% at 2 1/2 hours

Pentagastrin

6 mcg/kg/hr (iv)

100 mg/hr (iv)

60% at 1 hour

Caffeine

5 mg/kg/hr (iv)

300 mg (po)

100% at 1 hour

Insulin

0.03 units/kg/hr (iv)

100 mg/hr (iv)

82% at 1 hour

When food and betazole were used to stimulate secretion, inhibition of hydrogen ion concentration usually ranged from 45% to 75% and the inhibition of volume ranged from 30% to 65%.

2) Pepsin

300 mg of cimetidine tablets taken orally reduced total pepsin output as a result of the decrease in volume of gastric juice.

3) Intrinsic Factor

Intrinsic factor secretion was studied with betazole as a stimulant. Cimetidine tablets dosed at 300 mg orally inhibited the rise in intrinsic factor concentration produced by betazole, but some intrinsic factor was secreted at all times.

Other

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