When hay fever or hives strike, most people reach for one of three over-the-counter antihistamines: Zyrtec, Claritin or Allegra. All three are “second-generation” antihistamines, which means they cause much less drowsiness than older ones like Benadryl, and one dose lasts 24 hours. But they aren’t identical. Here’s how they compare.

The quick comparison

• Zyrtec (cetirizine 10 mg): often works fastest, usually within an hour. It’s the most likely of the three to cause drowsiness, though most people feel fine.
• Claritin (loratadine 10 mg): the least likely to cause drowsiness. It may take a little longer to kick in, typically 1 to 3 hours.
• Allegra (fexofenadine 180 mg): also very unlikely to cause drowsiness, and works within about an hour. Take it with water only. Fruit juices like grapefruit, orange and apple can stop it being absorbed properly.

What they all treat

All three relieve the same symptoms of hay fever and other upper respiratory allergies:

• Sneezing
• Runny nose
• Itchy, watery eyes
• Itchy nose or throat

All three also relieve itching from hives.

Which one should you choose?

• You need to stay sharp (driving, studying, operating machinery): Claritin or Allegra are the safest bets for avoiding drowsiness.
• You want the fastest relief: Zyrtec or Allegra.
• One didn’t work well for you: it’s worth trying another. People respond differently, and switching often helps.
• You drink a lot of fruit juice in the morning: Zyrtec or Claritin may be easier than Allegra.

Tips for getting the most out of allergy tablets

• Take it before symptoms peak. If you know pollen season is coming, start a daily tablet a few days before. Antihistamines work best at preventing symptoms.
• Take one at the same time each day during allergy season.
• Don’t double up. Never take two of these antihistamines together, or more than one dose in 24 hours.
• Cut your exposure. Keep windows closed on high-pollen days, shower and change clothes after being outdoors, and wash bedding weekly in hot water.

When to see a doctor

• Your symptoms don’t improve after a few days of treatment
• You have wheezing, shortness of breath or chest tightness, which may be asthma
• You have liver or kidney disease, or you’re pregnant or breastfeeding
• You’re 65 or older (for Zyrtec), or buying for a child under 6

Get emergency help if hives come with swelling of the lips, tongue or throat, trouble breathing, or dizziness.

Shop allergy relief

• Zyrtec (Cetirizine) 10 mg
• Claritin (Loratadine) 10 mg
• Alavert (Loratadine) 10 mg
• Allegra (Fexofenadine)

More in our Allergies guide: all our articles and products on this topic in one place.

This article is for general information only and isn’t a substitute for advice from a doctor or pharmacist. Always read the label before using any medicine.

Ozempic and Mounjaro are two of the most talked-about medicines of recent years. Both are once-weekly injections first approved to treat type 2 diabetes, and both can also lead to weight loss. They are not the same medicine, though, and neither is right for everyone. This guide explains how they work, how they differ, their side effects, and how to use them safely.

What is Ozempic?

Ozempic is the brand name for semaglutide, made by Novo Nordisk. It belongs to a group of medicines called GLP-1 receptor agonists. GLP-1 is a hormone your gut releases after you eat. Semaglutide copies its effects: it helps your pancreas release insulin when blood sugar is high, lowers the amount of sugar your liver releases, slows how fast your stomach empties, and reduces appetite.

Ozempic is approved in the US for adults with type 2 diabetes to improve blood sugar control, and to lower the risk of heart attack and stroke in adults with type 2 diabetes and heart disease. A higher-dose version of semaglutide is sold under a different brand name (Wegovy) specifically for weight management.

What is Mounjaro?

Mounjaro is the brand name for tirzepatide, made by Eli Lilly. It works on two hormone receptors instead of one: GLP-1 and GIP. This “dual” action is the main difference between the two medicines. Mounjaro is approved for adults with type 2 diabetes. Tirzepatide for weight management is sold under a different brand name (Zepbound).

Ozempic vs Mounjaro at a glance

• Active ingredient: Ozempic contains semaglutide. Mounjaro contains tirzepatide.
• How it works: Ozempic acts on GLP-1 receptors. Mounjaro acts on both GIP and GLP-1 receptors.
• How it’s taken: Both are injected under the skin once a week, in the stomach, thigh or upper arm.
• Dosing: Both start at a low dose that is raised step by step over several weeks. This helps the body get used to the medicine and reduces stomach side effects.
• Weight loss: In head-to-head research, tirzepatide has generally led to more weight loss than semaglutide, but results vary a lot from person to person.

Common side effects

Most side effects of both medicines affect the stomach and gut, and they are most common when starting or increasing the dose:

• Nausea and vomiting
• Diarrhoea or constipation
• Stomach pain, bloating or indigestion
• Reduced appetite
• Tiredness

Eating smaller meals, stopping when you feel full, avoiding greasy food and drinking enough fluids can help. Vomiting and diarrhoea can cause dehydration, which in turn can harm the kidneys, so tell your doctor if these don’t settle.

Serious side effects to know about

• Thyroid tumours: Both medicines carry a boxed warning because they caused thyroid C-cell tumours in rodents. It isn’t known whether this happens in people. They should not be used by anyone with a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia syndrome type 2 (MEN 2). See a doctor if you notice a lump in your neck, trouble swallowing, or a hoarse voice that doesn’t go away.
• Pancreatitis: Severe stomach pain that may spread to the back, with or without vomiting, needs urgent medical attention.
• Gallbladder problems: Including gallstones, especially with rapid weight loss.
• Low blood sugar: The risk is higher if you also take insulin or a sulfonylurea (such as glipizide or glimepiride).
• Eye changes: People with diabetic retinopathy should have their eyes monitored.
• Serious allergic reactions: Swelling of the face, lips or throat, or trouble breathing, needs emergency care.

Who should not use them?

• People with a personal or family history of MTC or MEN 2
• Anyone who has had a serious allergic reaction to semaglutide or tirzepatide
• People who are pregnant or planning a pregnancy, unless a doctor advises otherwise. Weight loss is not recommended during pregnancy.
• People with type 1 diabetes. These medicines are not a substitute for insulin.

If you have a history of pancreatitis, kidney problems, gallbladder disease, severe stomach problems (such as gastroparesis) or diabetic eye disease, your doctor needs to know before you start.

Important medicine interactions

Because both medicines slow stomach emptying, they can change how other tablets are absorbed. Mounjaro can make birth control pills less effective. Women taking the pill are usually advised to use a non-oral or backup method for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. Always give your doctor and pharmacist a full list of what you take.

Beware of fake Ozempic and Mounjaro

High demand has led to a large number of counterfeit and unapproved versions being sold online. Fake pens may contain the wrong dose, the wrong ingredient, or be contaminated, and people have needed hospital treatment after using them. Regulators, including the US FDA, have warned the public about counterfeit semaglutide and tirzepatide.

To protect yourself:

• Only use these medicines with a valid prescription from a licensed doctor.
• Get them from a licensed pharmacy that asks for your prescription. A seller that doesn’t ask for one is a warning sign.
• Check the packaging, label, lot number and expiry date, and compare them with the manufacturer’s information.
• Be cautious of prices that seem far too low, and of products described as “generic” Ozempic or Mounjaro.

Are they right for you?

Ozempic and Mounjaro can be very effective for the right person, but they are prescription medicines that need proper medical supervision. A doctor can check whether one of them suits you, choose the right dose, monitor your progress, and watch for side effects. They work best alongside healthy eating and regular physical activity.

Key takeaways

• Ozempic (semaglutide) works on one hormone receptor. Mounjaro (tirzepatide) works on two.
• Both are weekly injections approved for type 2 diabetes. Related brands of the same ingredients are approved for weight management.
• Stomach side effects are common, especially at the start. Serious side effects are rare but important to know.
• Only use them with a prescription, from a licensed pharmacy.

More in our Weight Management guide: including weight-loss medicines explained and how to lose weight safely.

This article is for general information only and isn’t a substitute for advice from a doctor or pharmacist. Don’t start, stop or change a medicine without talking to a qualified healthcare professional.