Stand in front of the allergy shelf and the boxes sort themselves into two camps: the ones that say "non-drowsy" in large letters, and the ones that don't. Some night-time cold and sleep products lean the other way and advertise the drowsiness as the point.
Both camps are antihistamines. The difference between them is real, it has a straightforward explanation, and it matters most in situations people don't always connect to an allergy tablet: driving, drinking, and the morning after. This guide covers the difference in general terms. It doesn't recommend a product or a dose; a pharmacist is the right person for that, because the right choice depends on you.
What antihistamines do
Histamine is a chemical your body releases as part of an allergic reaction. It's behind many of the familiar symptoms: sneezing, itchy and watery eyes, a runny nose, itchy skin and hives. MedlinePlus describes antihistamines as working by blocking the action of histamine.
Both drowsy and non-drowsy types do that. What differs is where else in the body they act.
First generation: the "drowsy" ones
The older antihistamines are often called first-generation antihistamines. The NHS gives chlorphenamine, diphenhydramine and promethazine as examples of the drowsy type.
Their best-known side effect is drowsiness. The NHS lists sleepiness along with reduced coordination, slower reactions and impaired judgement, and notes other side effects such as a dry mouth, blurred vision and difficulty passing urine.
That drowsiness is why some of these ingredients turn up in night-time cold products and sleep aids. MedlinePlus notes that diphenhydramine, for example, is used for allergy and cold symptoms and also for occasional sleeplessness in adults.
A few practical consequences:
- The effect can outlast the night. A drowsy antihistamine taken at bedtime may still affect you the next morning. Check the leaflet, and see how it affects you before relying on being sharp early the next day.
- It's often hiding in combination products. Night-time cold and flu remedies frequently include a sedating antihistamine. Read the active ingredients. If you're already taking a separate antihistamine, you may be doubling up. Our guide to common over-the-counter medication mistakes covers how to check.
- Some groups need more care. The NHS suggests asking a pharmacist about the best option if you're pregnant, breastfeeding, older, or taking other medicines.
Second generation: the "non-drowsy" ones
Newer antihistamines are often called second-generation. The NHS gives cetirizine, fexofenadine and loratadine as examples, and describes non-drowsy types as generally the best option because they're less likely to make you feel sleepy.
"Less likely" is the important phrase. The NHS lists drowsiness as a possible side effect of these too, along with headaches and nausea. Most people won't feel sleepy on them, but some do, and there's no way to know which group you're in until you've tried one.
So "non-drowsy" on a box is a description of how most people respond, not a promise about how you will.
Driving and machinery
This is where the difference matters most.
The NHS advises not to drive, cycle or use machinery after taking an antihistamine that makes you drowsy. MedlinePlus gives similar advice for diphenhydramine: don't drive or operate machinery until you know how the medicine affects you.
A few points that follow:
- You may not notice you're impaired. Slower reactions and poorer judgement aren't always obvious from the inside. Feeling "fine" isn't a reliable test.
- Try a new antihistamine at a time when you don't need to drive. That applies to non-drowsy ones too, since some people do get drowsy on them.
- Read the label for driving warnings. They're there for the specific product.
- Driving while impaired by medicine can be illegal. In the UK, GOV.UK says it is illegal to drive if you are unfit to do so because of legal or illegal drugs. If a medicine affects your ability to drive safely, you shouldn’t drive.
Alcohol
The NHS advises trying not to drink alcohol while taking an antihistamine, particularly a drowsy type, because it can increase the chance of it making you sleepy. MedlinePlus says the same about diphenhydramine: alcohol can add to the drowsiness.
The same caution applies to other things that cause drowsiness. If you take another medicine that makes you sleepy, ask your pharmacist whether it's sensible to add an antihistamine on top.
Choosing one
The NHS makes a point worth repeating: some people find certain antihistamines work well for them and others don't. It isn't just about drowsy versus non-drowsy; people respond differently to individual medicines.
A pharmacist can help you choose based on your symptoms, when you need relief, whether you drive, and what else you take. It's a good question to ask at the counter, and you don't need a prescription for the conversation.
If you take an antihistamine regularly, for example through a hay fever season, it can help to take it at a consistent time. Doz can hold a seasonal medication alongside your others, with reminders and a refill reminder while you still have some left, so you don’t run out mid-season, and you can use the days supply calculator to see how long a pack will last. For a medicine taken only as needed, the label's minimum gap between doses still applies; the next dose time calculator shows the earliest time for your next dose based on the interval on your label.
Frequently Asked Questions
What is the difference between drowsy and non-drowsy antihistamines?
Both block histamine, which drives allergy symptoms. Older, first-generation antihistamines such as chlorphenamine and diphenhydramine commonly cause sleepiness. Newer, second-generation types such as cetirizine, fexofenadine and loratadine are less likely to, although some people still feel drowsy on them.
Can I drive after taking a non-drowsy antihistamine?
Many people can, but not everyone. The NHS lists drowsiness as a possible side effect even of non-drowsy types. Try a new antihistamine when you don't need to drive, see how it affects you, and follow the driving warning on the label. Don't drive if you feel drowsy or your reactions are affected.
Can I drink alcohol with antihistamines?
The NHS advises trying not to, especially with a drowsy type, because alcohol can increase the chance of feeling sleepy. Check the leaflet for the specific product and ask your pharmacist if you're unsure.
Why are antihistamines in night-time cold medicines?
Some first-generation antihistamines cause drowsiness, which is why they're included in night-time and sleep products. That also means you could double up if you take a separate antihistamine. Check the active ingredients on each product and ask a pharmacist before combining them.
Which antihistamine is best for me?
That depends on your symptoms, when you need relief, whether you drive, and your health and other medicines. People also respond differently to individual antihistamines. A pharmacist can recommend an option for your situation.
Where to start
Next time you reach for an allergy or night-time cold product, check the active ingredients and find out which kind of antihistamine it contains, if any. If it's a drowsy type, plan around it: not before driving, not with a drink. If you're not sure which suits you, ask the pharmacist at the counter.
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