The Morning After: Four Beers, Four Hours' Sleep and an Antihistamine

Alcohol first. Four 500 ml beers at 5% is 100 ml of pure alcohol - roughly ten standard drinks. A workable rule of thumb for a 75 kg man is about 0.02% blood alcohol per standard drink, which puts the peak somewhere around 0.12–0.15%; spreading them over four hours and eliminating at roughly 0.015% per hour brings the peak down to something like 0.10%. From the last drink at 02:00 to 06:45 is about 4.75 hours, which removes only about 0.07%. That leaves an estimate in the region of 0.03–0.05% - right at or above the 0.05% limit, with wide individual variation. You cannot know which side of the line you are on, so for driving purposes you must assume the wrong side.

Then the medication. Chlorpheniramine is a sedating first-generation antihistamine. It peaks one to three hours after the dose - precisely when you would be driving - and it causes drowsiness, slowed reactions and degraded tracking. Its effect is additive with residual alcohol and with four hours' sleep.

Then fatigue. Four hours' sleep after drinking is not restorative sleep; you are sleep-deprived and hungover, dehydrated, and driving a fast mountain road first. Being 'fine, just tired' is the exact description of an impaired driver's self-assessment - the impairment removes the judgement needed to notice it.

Options, cheapest risk first:

• Let the flatmate drive - he did not drink and is not medicated. This solves the journey completely.
• Taxi or ride-hailing, or the intercity bus. Work out the fare against the real cost of a positive test: a heavy fine, licence suspension, possible detention, and a job that depends on the licence.
• Call in and start late, or work remotely for the morning. A two-hour delay changes the alcohol estimate materially; the medication effect still has to be considered, so re-assess rather than assuming.
• Do not 'sleep it off for 20 minutes' in the car and then drive. Short naps do not clear alcohol.

What to tell the flatmate: he is applying the wrong test. The question is not whether you feel paranoid or whether he thinks you seem normal - it is whether you can show you are under the limit and unimpaired, and you cannot. Exhaustion and a sedating antihistamine alone are enough to make you unfit even at zero alcohol, and 'riding along for company' does nothing about that; it just puts a second person in the car when it goes wrong. The uncomfortable conversation now is cheaper than the one after a checkpoint or a crash.

Cooperate, and do not try to manage it.

• The test is a lawful check. You comply, you stay polite, you produce your licence and vehicle documents if asked, and you do not argue at the roadside - disputes belong in the legal process afterwards.
• Refusing is itself an offence and is treated seriously: a refusal is generally read against you as evidence that you knew you would fail, and it does not avoid the suspension or the fine.
• Do not try to beat it. Gum, coffee, water, mouthwash and 'breathing deeply' do not change blood alcohol; trying them only makes you look like you are obstructing.
• Disclose the medication if asked about drugs or medicines. Driving while impaired by a medicine is an offence on its own, with or without alcohol; the label on chlorpheniramine warns against driving.
• If you are over the limit: expect a substantial fine, suspension or withdrawal of the licence, and possible detention, plus the practical consequences - if you crash while impaired, the insurer can refuse or reclaim, and you carry the conviction into future employment and licence checks.

The real point: the decision was taken at 06:45 at your front door, not at the checkpoint. Once you are behind the wheel, every option left is a bad one.