Histamine acts on several different receptor types. In the skin, the H1 receptor does most of the work: it drives the wheal, the flare, and the itch, and it is what cetirizine and the other allergy antihistamines block.
But the small blood vessels in the skin also carry H2 receptors, and histamine acting there contributes to the vessel widening and leakage behind a hive. A drug that only blocks H1 leaves those alone. Famotidine blocks H2. Adding it means both receptor types are covered.
That is the theory, and it is a reasonable one. What happens in practice is less impressive. Most of the hive is H1-driven, so blocking H2 adds only a fraction of what is left. Trials of adding an H2 blocker to an H1 antihistamine for hives have been small, mixed, and not well designed, and guidelines describe the evidence as low quality. Some people do notice a difference; many do not.
The same drug reduces stomach acid because the acid-producing cells in the stomach lining are switched on through H2 receptors. That is its approved job, and it does it well.
Kidney impairment requires a lower dose. Alcohol is not a direct interaction, but it aggravates the reflux famotidine is usually treating.
No routine blood tests are needed. If you have reduced kidney function, your prescriber will use a lower dose and may check kidney numbers, since famotidine is cleared by the kidneys.
For heartburn, a dose works within an hour or so.
For hives, do not expect a same-day transformation. It is an add-on, and its contribution is modest, so give it one to two weeks alongside your regular antihistamine and judge whether the overall picture is better. If it is, keeping it is reasonable. If nothing has changed, stopping it is equally reasonable — there is no benefit to carrying a second daily tablet that is not earning its place.
If it was helping your hives, they may drift back toward where they were. Because the effect is modest to begin with, many people stop it and notice nothing, which is itself useful information.
If you have been taking it for reflux or ulcer symptoms at higher doses for a long stretch, acid symptoms can rebound briefly after stopping, so a doctor may suggest tapering rather than stopping outright.
There is no withdrawal itch phenomenon with famotidine of the kind reported after long-term daily cetirizine or levocetirizine. Do not stop your H1 antihistamine at the same time as your famotidine if you are testing whether the famotidine is doing anything — change one thing at a time.