Are antihistamines safe?
All medicines have side effects, but some have very
few, mostly mild, whilst others have a lot, mostly serious.
The latest antihistamines are amongst the safest medicines
there are. But there are differences between them in side effects.
Older antihistamines are bad news unless you need help
getting to sleep
Older antihistamines do have more side effects. The most
serious of these are both sleepiness, and a harmful effect on driving, learning
and similar tasks even when they don't seem to make you feel sleepy. They affect
your brain in the same way as alcohol.There is no real
doubt in the minds of researchers that older antihistamines can cause road
accidents.
They also hinder learning, exam performance and skilled
tasks.
Older antihistamines also had some other side effects which
might in a few people cause difficulty passing water or increased pressure in
the eyes (glaucoma), although this seems to have been rare.
There is now little reason to use the older antihistamines.
How do we know that the new antihistamines
are better? There has been lots of research which shows this, but two
experiments are specially interesting.
Road safety experiment
In one set of experiments, drivers had to
drive along a motorway for 100 kilometres. They had to drive at a steady speed
and keep as near to the middle of the slowest lane (the left hand lane, as it
was in the Netherlands) as they could. A video camera on the roof of the car
filmed the white line on the road. When they got back, the video tape was
played, and the movement of the white line on the screen was measured.
In some tests the drivers took an old
antihistamine, and in other tests they took a new one or a dummy tablet.
The old antihistamines made the drivers
wobble more during driving, sometimes to a dangerous extent, so that the test
drive had to be stopped. (They used dual control cars, and had permission to
stop on the hard shoulder of the road when this happened.) Old antihistamines
were frankly dangerous.
New antihistamines caused no increase in
wobbling compared to the dummy tablets.
Alcohol, even within the legal limit for
driving, also made steering more wobbly. But the new antihistamines together
with alcohol were no worse than the alcohol alone.
So the new antihistamines are much safer
for drivers.
Learning experiment in children with
hayfever
Schoolchildren with hayfever were divided
into three groups. For the experiment, during the hayfever season, children in
the first group took dummy tablets. Those in the second group took an old
antihistamine. Children in the third group took a new antihistamine, Triludan.
All the children then sat in front of
computers. They used a computer program which taught them about farming in the
Sahel Desert. As the children were from the town of Maastricht in the
Netherlands, this meant they knew nothing about this at the beginning.
When the children had finished learning,
they played a computer game. Guess what? The game was about - farming in the
Sahel desert. Depending how well they did, each child got a score.
Here are the results. The children whose
hayfever was treated with the old antihistamine did WORSE than the ones who just
got dummy tablets. The sleepiness, or inattentiveness did more harm to how they
did than hayfever alone.
But the children who took the new
antihistamine, Triludan, did BETTER than the children who took dummy tablets.
This shows that hayfever by itself harms learning, and that treating the
hayfever undoes this harm, provided the treatment does not make the children
sleepy.
Both sets of experiments were done by the
team of Professor O'Hanlon, of Maastricht, Netherlands. If he'd let me use some
photographs of these experiments, I would show them to you on the web.
Many other experimens have shown similar
results.
So the new antihistamines are much better for
schoolchildren, students, and people who need to use their brains, which means
all of us.
Two of the newer antihistamines do have a rare but serious
problem Two of the newer antihistamines (terfenadine
and astemizole) have on rare occasions had life-threatening or fatal side
effects. In overdose or when combined with one of a small number of other
medicines, or in people with certain kinds of heart trouble, they could cause
the heart to beat in an abnormal way, and over enormous numbers of people who
took the drug, a small number of people died. With proper precautions
terfenadine
and astemizole are still very good and safe drugs. But in the case of
terfenadine even that problem has now been solved. In the body, terfenadine is
changed into something else which has the good effects of terfenadine but not
the bad effects. So the obvious answer was to put the something else into a
tablet. This has now been done. The new medicine is called Telfast in Britain (Allegra
in the USA), and its official name is fexofenadine. Fexofenadine has the good
effects of terfenadine but not its very rare serious side effect.
So the bottom line is: some of the latest antihistamines are among the safest
medicines there are. If they relieve a real illness, it is reasonable to take
them, if necessary over long periods, without worrying.
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