It is an offence to drive while impaired by drugs, and that includes medicines bought over the counter. "It was only a hay fever tablet" is not a defence if you were impaired.
The offence is about impairment, not about whether a substance was prescribed. A medicine legally bought in a supermarket can still put you over the line.
Which Antihistamines Are the Problem
First-generation — genuinely sedating
- Chlorphenamine (Piriton)
- Promethazine (Phenergan)
- Diphenhydramine, found in many night-time and combination products
These cross into the brain readily and cause significant drowsiness and slowed reactions. They should not be combined with driving. The sedation can also persist into the following morning, which is the part people miss — a night-time dose can still be affecting you at breakfast.
Second-generation — "non-drowsy", with caveats
- Fexofenadine — the least sedating; clinical trials show no more drowsiness than placebo
- Loratadine — rarely sedating at standard doses
- Cetirizine — classed as non-drowsy, but the most sedating of the three, with drowsiness reported by roughly one in ten to one in twenty users
The MHRA advises caution with driving and machinery until you know how cetirizine affects you. "Non-drowsy" describes a population average, not a promise about you specifically.
Practical Rules
- Try a new antihistamine on a day you are not driving. This is the whole answer, and almost nobody does it.
- Do not combine with alcohol. The effects compound.
- Check combination products. Cold and flu remedies, night-time formulations and some travel sickness tablets contain sedating antihistamines under names you may not recognise.
- If you feel dulled, do not drive. Feeling "fine but a bit slow" is exactly the state the law is about.
Hay Fever Itself Impairs Driving
The symptoms are a road safety issue in their own right. Violent sneezing fits close your eyes for a moment at speed, streaming eyes blur vision, and poor sleep from nasal congestion reduces concentration.
Untreated hay fever is not the safe option. Well-treated hay fever with a genuinely non-sedating antihistamine is safer than either extreme.
If You Drive for a Living
The stakes are higher and so is the scrutiny. Fexofenadine is generally the safest choice on sedation grounds, and a steroid nasal spray adds symptom control with negligible systemic effect. Talk to a pharmacist about your specific combination rather than picking on price alone.
Some employers have their own policies on medication and safety-critical work. Check rather than assume.
The Cost Angle
Fexofenadine 120mg is available over the counter and own-brand versions are inexpensive. If sedation matters to you — and it should if you drive — the marginal cost over cetirizine is small, and it is the clearest case in this category where the slightly dearer option is the right one.