Reactine allergy medication contains cetirizine dihydrochloride, a second-generation antihistamine widely used to relieve hay fever, allergic rhinitis, and urticaria. In the UK, the same active ingredient is sold under brand names such as Piriteze and Zirtek, as well as in generic forms. Understanding exactly what Reactine contains — including its active ingredient, inactive excipients, how it works, and who should use it with caution — helps patients make informed decisions about their allergy treatment. This article covers the full ingredient profile, recommended doses, possible side effects, and important safety considerations in line with MHRA, NICE, and BNF guidance.

What Cetirizine Dihydrochloride Contains and How It Works

Reactine is a brand of antihistamine medication containing the active ingredient cetirizine dihydrochloride, a second-generation antihistamine. In the UK, cetirizine dihydrochloride is also sold under other brand names, including Piriteze and Zirtek, as well as in generic (unbranded) forms. Standard tablets contain 10 mg of cetirizine dihydrochloride, and a liquid oral solution (1 mg/ml) is also available, which is particularly suitable for children and those who have difficulty swallowing tablets.

In addition to the active ingredient, tablets and oral solutions contain a number of inactive excipients — ingredients that help form the tablet, provide stability, or create the coating. These vary between brands and formulations. Common excipients in some cetirizine tablet products include lactose monohydrate, maize starch, colloidal anhydrous silica, magnesium stearate, hypromellose, macrogol, and titanium dioxide (E171). Because excipients differ between products, patients should always check the patient information leaflet (PIL) supplied with their specific product for a full and accurate list.

Cetirizine dihydrochloride works by selectively blocking peripheral H1 histamine receptors. When the body encounters an allergen — such as pollen, pet dander, or dust mites — it releases histamine, a chemical that triggers classic allergy symptoms including sneezing, itching, a runny nose, and watery eyes. By blocking these receptors, cetirizine prevents histamine from binding and reduces or eliminates these symptoms.

Unlike first-generation antihistamines such as chlorphenamine, cetirizine has low penetration of the blood-brain barrier, which means it is considerably less likely to cause significant drowsiness. This makes it suitable for daytime use for most people, though some individuals may still experience mild sedation. Cetirizine dihydrochloride products are licensed in the UK by the Medicines and Healthcare products Regulatory Agency (MHRA); prescribers and patients can find full product information in the Summary of Product Characteristics (SmPC) available on the electronic Medicines Compendium (emc).

Recommended Doses and How to Take Cetirizine Safely

Cetirizine dihydrochloride should always be taken in accordance with the patient information leaflet supplied with the product, or as directed by a pharmacist or GP. The standard recommended doses, as set out in UK SmPCs, are as follows:

• Adults and children aged 12 years and over: One 10 mg tablet (or 10 ml oral solution) once daily

• Children aged 6 to 11 years: 5 mg twice daily — use 5 ml of the 1 mg/ml oral solution twice daily, or half a tablet twice daily only if the tablet is scored and the PIL confirms it may be divided

• Children aged 2 to 5 years: 2.5 mg (2.5 ml of oral solution) twice daily — always use the appropriate liquid formulation

For paediatric doses, the oral solution is generally preferred to ensure accurate measurement. Tablets should only be halved if they are scored and the product's PIL explicitly permits this.

Tablets can be taken with or without food and should be swallowed whole with a glass of water. The oral solution should be measured carefully using the dosing device provided. Taking cetirizine at the same time each day helps maintain consistent symptom control, particularly during high-pollen seasons or periods of prolonged allergen exposure.

Renal impairment: Because cetirizine is primarily excreted by the kidneys, dose adjustment is required in adults with reduced kidney function. As a general guide (refer to the BNF or SmPC for full detail): a dose of 5 mg once daily is typically recommended if creatinine clearance (CrCl) is between 10 and 49 ml/min; cetirizine is contraindicated if CrCl is below 10 ml/min. Patients with known kidney problems should seek advice from their GP or pharmacist before use.

Hepatic impairment: Dose adjustment is not usually required for hepatic impairment alone; however, patients with both hepatic and renal impairment should seek medical advice.

For seasonal allergic rhinitis (hay fever), treatment may be started just before the expected pollen season and continued throughout. For perennial (year-round) allergic rhinitis or chronic urticaria (hives), longer-term use may be appropriate under the guidance of a healthcare professional. NICE Clinical Knowledge Summaries (CKS) for allergic rhinitis and urticaria support the use of non-sedating antihistamines such as cetirizine as a first-line treatment for these conditions. If symptoms of allergic rhinitis persist despite antihistamine use, a healthcare professional may recommend adding an intranasal corticosteroid.

It is important not to exceed the stated dose. If a dose is missed, it should be taken as soon as remembered — unless it is nearly time for the next dose, in which case the missed dose should be skipped. Doubling up doses is not recommended. If symptoms remain poorly controlled despite regular use, patients should consult their GP or pharmacist for further assessment.

Possible Side Effects and Precautions

Cetirizine dihydrochloride is generally well tolerated, but like all medicines, it can cause side effects in some people. Most side effects are mild and transient. The most commonly reported include:

• Drowsiness or somnolence — although less sedating than older antihistamines, some individuals may still feel sleepy

• Headache

• Dry mouth

• Dizziness

• Fatigue

• Gastrointestinal discomfort, such as nausea or abdominal pain

Because cetirizine can cause drowsiness in some users, patients are advised to assess their individual response before driving or operating machinery. The MHRA advises caution in this regard, particularly when starting treatment for the first time. Further UK-specific guidance on medicines and driving is available at GOV.UK.

Alcohol: UK SmPC data do not demonstrate a clinically significant potentiation of cetirizine's effects at low alcohol levels; however, additive sedation is possible. Patients should use caution if drinking alcohol during treatment. Similarly, taking cetirizine alongside other central nervous system depressants — such as sedatives or anxiolytics — may increase the risk of drowsiness.

Some formulations of cetirizine tablets contain lactose. Individuals with rare hereditary conditions such as galactose intolerance, total lactase deficiency, or glucose-galactose malabsorption should check the PIL for their specific product and discuss alternatives with their pharmacist or GP if lactose is listed as an excipient.

In rare cases, cetirizine may cause more serious reactions, including urinary retention, which is of particular concern in men with prostate enlargement. There have also been very rare reports of hypersensitivity reactions, including angioedema (swelling of the face, lips, or throat) and anaphylaxis. If any of these occur, the medication should be stopped immediately and emergency medical attention sought (call 999).

Patients should always read the full patient information leaflet and report any unexpected or concerning symptoms to their healthcare provider. Suspected side effects should be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk or via the Yellow Card app.

Who Should Avoid Cetirizine or Seek Medical Advice First

Whilst cetirizine dihydrochloride is suitable for many adults and children, certain groups should exercise caution or consult a healthcare professional before use.

Do not take cetirizine if you:

• Are allergic to cetirizine dihydrochloride, levocetirizine, hydroxyzine, other piperazine derivatives, or any of the excipients listed in the PIL for your specific product

• Have severe kidney disease (end-stage renal impairment with creatinine clearance below 10 ml/min)

Seek medical advice before use if you:

• Have mild to moderate kidney impairment — a dose reduction is likely to be necessary (see dosing section above)

• Have epilepsy or a history of seizures, as there is some evidence that antihistamines may lower the seizure threshold in susceptible individuals

• Have difficulty passing urine, such as due to an enlarged prostate or spinal cord problems, as cetirizine may cause urinary retention

• Are pregnant — NHS guidance indicates that cetirizine can be used during pregnancy when clearly needed; however, it is good practice to discuss use with a GP or pharmacist, particularly in the first trimester

• Are breastfeeding — cetirizine passes into breast milk in small amounts. Whilst this is unlikely to cause harm to a healthy infant, loratadine is often preferred as the antihistamine of choice when breastfeeding. Breastfeeding mothers should consult their GP or pharmacist before use and monitor the infant for any unusual drowsiness

Children under 2 years of age should not be given cetirizine without specific medical advice. Licensed age indications vary by formulation; always check the PIL or SmPC for the specific product being used.

Older adults may be more susceptible to side effects such as dizziness and sedation, which can increase the risk of falls. A GP or pharmacist should be consulted if cetirizine is being considered for an elderly patient, particularly those taking multiple medications.

If allergy symptoms are severe, worsening, or not responding to cetirizine after a reasonable period of use, patients should seek a GP review. For chronic urticaria (symptoms persisting for more than six weeks), a GP may consider up-dosing under medical supervision or referral to a specialist for further assessment and management, in line with NICE CKS guidance on urticaria. This is particularly important if symptoms could indicate a more serious underlying condition requiring specialist care.

Frequently Asked Questions