Hydroxyzine blocks the H1 histamine receptor, which is the same receptor cetirizine, loratadine and fexofenadine block. In hives, where histamine released by mast cells is genuinely driving the wheals and the itch, that action is the point and it works well.
In eczema and most other itchy rashes, histamine is not the main signal. The itch is driven by other pathways entirely, which is why antihistamines are often disappointing for eczema and why doubling the dose does not help.
What makes hydroxyzine different from the newer antihistamines is that it crosses into the brain, where the same H1 receptors are involved in staying awake. Blocking them there causes drowsiness. For someone scratching themselves raw at 3am, that is the useful effect — sleep interrupts the scratching, the skin stops being damaged, and the rash gets a chance to settle. Naming that honestly matters, because it explains why hydroxyzine helps eczema at night and does almost nothing for daytime itch beyond making you tired.
Driving and machinery. Do not drive the morning after until you know how long the drowsiness lasts for you. Next-day impairment is real even when you feel awake, and it is the most common practical problem with this drug.
Alcohol. The sedation compounds. Avoid it on nights you take hydroxyzine.
Other sedating medicines — sleeping tablets, opioid painkillers, benzodiazepines, muscle relaxants, some antidepressants — add to the effect and to the risk of breathing problems in combination.
Drugs that affect heart rhythm. Hydroxyzine can prolong the QT interval, and combining it with other QT-prolonging drugs raises the risk of a dangerous rhythm. Tell your prescriber everything you take.
Other anticholinergic drugs — bladder medicines, some Parkinson's drugs, older antidepressants — add up to dry mouth, constipation, blurred vision, urinary retention and confusion, particularly in older adults.
No interaction with sun, and no food restrictions.
No routine blood tests for ordinary short-term use.
An ECG is worth considering before starting in anyone with known heart rhythm problems, a family history of sudden cardiac death, or who is already taking other drugs that prolong the QT interval. In everyone else this is not routine.
What actually needs following up is simpler: whether it is helping the sleep, and whether you are functional the next day. If the answer is no on either count, the dose or the drug should change rather than continue by default.
In older adults, the useful monitoring is for confusion, unsteadiness and falls, which is the reason many prescribers avoid it in this group.
The sedation starts within 15 to 30 minutes and peaks in about two hours. The first night usually tells you whether it is going to help you sleep.
For hives, the antihistamine effect on itch and wheals is noticeable within an hour.
For eczema and other itchy rashes, do not expect the rash to change. What changes over the first three to seven nights is the amount of scratching, and it is the reduced scratching that lets skin heal — so improvement in the skin follows a week or so behind the improvement in sleep.
The grogginess usually eases over the first week as you get used to it. If it has not eased by then, a lower dose or an earlier bedtime dose is the fix.
If a week of it has changed nothing at all about the itch, that is genuinely useful information. It means histamine is not driving the itch, and the answer lies in treating the skin — topical steroids, moisturiser, or a systemic treatment for the underlying condition — rather than in a stronger antihistamine.
Hydroxyzine can be stopped whenever you like after short-term use. There is no taper and no physical withdrawal.
What comes back is the itch, because nothing about the underlying condition changed. If the skin has healed during the sedated nights, the itch is often less than it was; if the eczema or hives are still active, it returns at full strength the first night without it.
After several weeks of nightly use, some people find their sleep is disrupted for a few nights after stopping. This settles within about a week.
The body also becomes used to the sedation over a few weeks, so it tends to make you less sleepy over time — which sounds like a benefit but usually means it is doing less of what you were taking it for. That is one argument for using it in bursts during flares rather than every night.