Many of us take medication, both for our little ones and for our major health problems.
Some of these compounds, however, come with side effects that can sometimes be serious.
This week Science Matters asked why drugs sometimes cause side effects.
To find out, we were able to count on the expertise of Nicolas Bertrand, professor at the Faculty of Pharmacy at Laval University.
Side and adverse effects
It is first necessary to distinguish between a side effect, which will not necessarily be bad, and an undesirable consequence.
An example of a side effect is the drug developed in the 1980s to treat benign prostatic hyperplasia, finasteride, which has also been shown to be effective against hair loss.
On the contrary, adverse effects lead to a negative consequence, including some cancer treatments that cause hair loss.
According to Nicolas Bertrand, the adverse effects of drugs can be classified into three main categories.
The first occurs when our immune system responds disproportionately to taking a drug, such as when a child develops a rash after being given an antibiotic. This is then referred to as an allergic reaction.
“We can’t really avoid these kinds of side effects because they are independent of the dose of the drug,” explains Nicolas Bertrand.
Side effects can occur after one stops taking a drug and it is accompanied by withdrawal symptoms, for example nausea and muscle cramps. This is particularly the case with pain medication or certain antidepressants.
“If you stop taking certain antidepressants, you have to gradually reduce the dose if you don’t want to have significant symptoms,” explains the professor.
Other undesirable effects manifest themselves more or less strongly depending on the quantity of drugs administered.
A good example is the adverse effects of first-generation antihistamines, such as Benadryl. These allergy medications are notorious for causing drowsiness, an effect that increases with the dose administered.
Same protein, different functions
Many side effects occur because the target of the drug may have different functions in different places in the body. By blocking its action, we will have the desired effect, but also an undesirable impact.
This is particularly what happens in the case of opiates used against pain.
In the brain, this drug blocks pain receptors. In the gastrointestinal tract, however, the molecule inhibits peristaltic movements, responsible for moving food forward through digestion and, eventually, the excretion of faeces. This is why opiates constipate those who use them.
The same phenomenon also explains why first-generation antihistamines, such as Benadryl, cause drowsiness.
“The histamine receptor on white blood cells is involved in allergic reactions, so when you take an antihistamine, you block the allergic reaction,” says Nicolas Bertrand. However, these receptors are also present in the brain and when the drug blocks their functioning, it causes drowsiness.
Since it was realized that these allergy drugs induce sleep, second-generation antihistamines that do not cause drowsiness – they hardly penetrate the brain – have been developed.
Tools not always precise
Other drugs cause side effects because their action is not specific enough.
For example, if a molecule made synthetically in the laboratory targets a given protein in the body, it can also disrupt the proper functioning of another very similar protein.
“Two proteins can have very different functions, but from the point of view of their physico-chemical particularities, their structure, they can be very similar. Sometimes we want to target one and not the other, but the molecule we have is not specific enough to act only on the target of interest”, explains Mr. Bertrand.
To illustrate his point, we can imagine that our medicine is a slotted screwdriver. Although this tool will be able to tighten slotted head screws, it will also be able to tighten some Phillips head screws.
When the drug is released into the body, so it wanders randomly and will tighten the slotted head screws, which will have the desired effect, but also some Phillips head screws, which may cause undesirable effects.