This is quite complex. Broadly speaking there are 4 reasons why people get side effects. As ever, people are not simple, so if you get side effects it could be due to all four reasons combined.
Over-diagnosis and over-treatment
We know that high blood pressure diagnosed in clinical settings leads to a high rate of false positives. False positives are when your readings are high when measured, but that you don’t actually have high blood pressure the rest of the time.
For this reason, it’s recommended that high blood pressure is diagnosed either by “ambulatory blood pressure monitoring” or by keeping a blood pressure diary at home. Ambulatory blood pressure monitoring is where you have a portable machine strapped to your arm and it measures your blood pressure every 15-20 minutes through the day and night. In the UK there is a standard protocol for keeping a blood pressure diary – but it’s not well known.
Either of these tests is better than taking a one-off reading because they have a lower rate of false positive. This means that if your average is high then it’s more likely that you do have high blood pressure.
If you don’t have high blood pressure, and it is treated, then you will end up with a lower than normal blood pressure. For may people this won’t matter, but for some, it will lead to side effects of feeling dizzy and nauseous etc.
This is why it’s so important to use accurate way to assess people’s average blood pressure both for diagnosis and to assess treatment.
Using high doses of single medications rather than a combination of low dose medications
This is because many blood pressure medications have side effects that are dose dependent. This means that you’re more likely to get side effects at higher doses. However, their benefits sometimes are not. This means that in some cases increasing the dose of a medication means that you’re more likely to get side effects, without necessarily getting any benefit.
Blood pressure medications also have an effect called “synergy”. This is where two or more different medications when combined have an overall beneficial effect greater than you’d expect. For this reason, often you’re better off on low doses of multiple medication types combined, than on a single agent at a high dose.
However, just to make things more complicated, the medications that work on the ACE system tend to have more beneficial effects at higher doses.
The Nocebo effect
The Nocebo effect is when people get side effects because they are expecting them. It’s the same effect as the placebo effect, but just in reverse.
If you think about it, it makes sense. The brain and the body are not separate. We know that if you think about a symptom a lot, then it makes that symptom worse. Conversely, we know that if you’re distracted from it, it makes the symptom better. It stands to reason that if you’re worried about side effects then you’re more likely to focus on them if they occur.
Things that make the nocebo effect worse are when someone has been coerced into having treatment. This includes high-pressure tactics such as scaring people into taking medication. This also includes when they feel that they have little understanding of why they are taking their medication
Things that make the nocebo effect better are when people are well informed about side effects. That way when they occur, people don’t get so worried, and often the side effects settle.
Another thing that reduces the nocebo effect is when people are well informed about the reason that they’re taking the medication and clearly understand the benefits.
So, in short, you’re much less likely to have side effects if you’re making the decision yourself, without coercion, and understand exactly what the benefits of taking it are.
Finally, some people get side effects, because all meds carry with them some risk of side effects, and through random chance, some people just seem to be more prone than others to them.
However, often with blood pressure medication, there is plenty that can be done to reduce or minimise them.