On long motorway journeys, the effect of some basic remedies, including cold and hay fever cures, could have the same effect as being drunk behind the wheel.
But company drivers taking over-the-counter and prescribed drugs containing antihistamines fail to realise the serious effect they can have on their judgement on the road.
At the annual Fleet Safety Forum staged in London by Brake, the road safety charity, delegates were told it was vital for clear warnings to be put on medicine bottles.
Prof Ian Hindmarch, head of the human psycho-pharmacology research unit at the University of Surrey, said: 'The drugs prevent people from judging whether they are fit to drive. We must find a way of informing them how easily they can slip from the twilight point of nodding off, to going to sleep at the wheel.'
A study has shown the dangers of taking promethazine, one of the anti-histamine drugs, which is used to sedate patients before medical procedures.
He said: 'It is worse than alcohol at the legal limit at increasing the distance travelled at 70mph before braking. This is a serious issue, especially because some people take more than the suggested dose.
'Advice tends to be on a leaflet in the box, in writing so small you need a magnifying glass to read it. A red triangle on the packet would be best, as it's an internationally recognised danger symbol.'
Many drivers take antihistamines to overcome hay fever, eye and ear inflammation, skin irritation, insomnia and sleep disturbance.
Without them, a driver doing 70mph could drive blind on motorways because it is almost impossible to keep the eyes open when sneezing.
'That's quite terrifying,' said Hindmarch, 'but antihistamines close down receptors that make you sneeze and make you sleepy. It is like a mild anaesthetic, you can't fight it and this is why we are concerned.'
Research in 1985 showed 5% of people take antihistamines and the proportion is likely to have risen.
Hindmarch told the forum, sponsored by Peak Performance and hosted by BT: 'They are not benign, because they create a state of stupor.'
Blood samples were taken from patients admitted to a London hospital emergency unit to try to establish to what extent drugs or alcohol had been to blame for accidents of different kinds.
Drugs including antihistamines and anti-depressants were shown to be more to blame than alcohol at just below the legal limit.