Which one of the following is used as Antihistamine?

First, let us recall the basic definition: an antihistamine is a drug that blocks the action of the biochemical messenger histamine in the body, thereby reducing symptoms such as sneezing, itching, watery eyes, and other allergy-related effects.

Now we examine each option one by one and match it with its therapeutic class.

We have Option A, Omeprazole. Omeprazole is well known as a proton-pump inhibitor. It suppresses gastric acid secretion by specifically inhibiting the $$H^{+}/K^{+}$$-ATPase pump present in the stomach lining. Since its action is on gastric acid production and not on histamine receptors, Omeprazole is not an antihistamine.

Next we look at Option B, Chloramphenicol. Chloramphenicol is a broad-spectrum antibiotic that inhibits bacterial protein synthesis by binding to the $$50\text{S}$$ ribosomal subunit in bacteria. Because antibiotics target microorganisms and not histamine receptors, Chloramphenicol is also not an antihistamine.

Then we consider Option C, Diphenhydramine. Diphenhydramine belongs to the class of first-generation $$H_1$$-receptor antagonists. In simple words, it blocks histamine $$H_1$$ receptors, thereby preventing histamine from exerting its usual effects such as vasodilation, increased capillary permeability, and sensory nerve stimulation. Because blocking histamine is precisely the function that defines an antihistamine, Diphenhydramine fits the required category perfectly.

Finally, Option D, Norethindrone, is a synthetic progestin used in hormonal contraceptives. Its pharmacological action involves mimicking the hormone progesterone; it has nothing to do with histamine blockade. Therefore, Norethindrone is not an antihistamine.

Comparing all the options with the definition of an antihistamine, only Diphenhydramine (Option C) satisfies the criterion.

Hence, the correct answer is Option C.