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Adverse effects: Knowing more can help you make the best decisions for your treatment

Created: ; Last Update: February 13, 2013.

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We all have had this experience – or know someone who has had it: although a therapy has helped, the adverse effects (also called side effects) were so bothersome or severe that in the end the treatment was hardly worthwhile. Or a therapy might be very important for us, but the medical information we are given has a long list of side effects, which can easily put us off or even frighten us.

In these cases we sometimes do not quite know what to do. You might even feel that it would be better not to know about all the things that could happen in the first place! And yet knowing about side effects can be important for you to get the best health care – even though it is not always easy to weigh up the advantages and disadvantages before making a decision.

This article tries to help you understand information on side effects, put them into the right perspective, and deal with the risks that medications, surgery, dietary supplements or other health interventions may have. We will talk a lot about “therapies” or “treatments”. But many of the problems discussed also apply to diagnostic examinations, health advice and all other health-related habits. To get a overview of the topic “Choosing between treatment options”, click here.


The starting point: What adverse effects might a certain treatment have?

One thing should be clear right from the start: Anything that has a benefit can also have an adverse effect. In other words: if a therapy can help, it can also be harmful. It also usually follows that if something does not have any side effects at all, it is also very unlikely to help.

There are different ways a body can react to a medication or a treatment. Some reactions occur immediately, for example when you get a shot and your skin gets red and itchy where you were injected. Your body might also show what is called a systemic or generalized reaction, for example if you feel sick or dizzy. Some medications can have an impact on a single organ such as the liver. A therapy might also make an illness worse instead of making it better, or cause symptoms that are similar to the ones of the illness being treated. This is the case, for example, when a flu medication causes a headache.

Side effects are also sometimes called adverse effects, adverse drug reactions (ADR), adverse reactions, adverse events or adverse experience. People can also be allergic to certain medications. Although technically all these terms have slightly different meanings, they essentially mean the same thing to patients: the therapy does some kind of harm.

How can I tell what impact an adverse effect could have on me?

What is an acceptable risk for one person can be unacceptable for another. It can also depend on the health problem to be treated. For example, most people would accept the severe side effects of chemotherapy for treating cancer – whereas equally severe adverse effects in a medication for mild headache naturally would not be acceptable. In an unbearably painful migraine on the other hand, this could again be a different story.

The following questions can help you judge the significance an adverse effect could have for you:

  • What exactly is the adverse effect? Adverse effects are often described using words you have never come across before, or vague medical terms that can mean almost everything – from mild dizziness to fainting spells. You can find many medical terms explained in our dictionary.
  • When will it most probably occur? The adverse effect might occur the moment you start therapy or develop later on. It could happen in the morning or at night. All these things might be important for you in evaluating how an adverse effect might affect your life and wellbeing.
  • How long will it last? Adverse effects are often “transient” (temporary). This means that they will go away on their own. Sometimes adverse effects are there for some time, until your body has gotten used to the therapy. But some may also last for the entire duration of the treatment – and stop as soon as the therapy has ended. Other adverse effects are permanent, or do not occur during therapy but can develop as a delayed effect.
  • Can there be interactions you should know about? Therapies themselves can lead to adverse effects, but they can also interact with other treatments or with something you do (drinking alcohol, for example, or taking medications with milk or certain juices) and cause harm or make a treatment less effective.

Once you have found answers to these questions you will probably have a better idea about which side effects you really consider significant and which ones might not have a big impact regarding the possible benefit of the therapy.

How often do adverse effects occur?

Getting precise information about the frequency of adverse effects can be difficult for researchers too. There are several reasons for this: Treatment trials are typically set up in order to find out whether a therapy has the desired effect. The best way to find this out are randomized controlled trials. In this kind of trial very similar groups of people receive different treatments; and researchers then compare what happens. You can read more about why trials are carried out this way here.

Adverse effects that occur often enough can also be observed in these trials. But often the trials are not big enough or do not go on for long enough to register precisely how often adverse effects occur that are not as common or that only show after the treatment has been used for a longer period of time. Many types of organ damage or cancers, for example, take years or even decades to develop. In addition, people who are particularly vulnerable and/or who would have particularly severe consequences from adverse effects are usually excluded from trials. Therefore important groups like children, pregnant women, very old people or people who have several diseases at the same time, are often not involved enough in these trials.

If there are limitations like the ones described, other additional scientific methods are normally used in order to monitor the safety of a treatment. However, these other methods do usually not allow conclusions about whether the alleged “effect” really is due to the treatment.

For many people it is easier to assess side effects if they read numbers on their frequency. In order to make this information consistent and comprehensible, package inserts for medications in Europe, for example, use the classification of the European Medicines Agency (EMA):

  • very common: could happen in 1 or more out of 10 people who use this therapy (10% or more)
  • common: could happen in 1 to 10 out of 100 people who use this therapy (1 to 10%)
  • uncommon: could happen in 1 to 10 out of 1,000 people who use this therapy (0.1 to 1%)
  • rare: could happen in 1 to 10 out of 10,000 people who use this therapy (0.01 to 0.1%)
  • very rare: could happen in fewer than 1 out of 10,000 people who use this therapy (less than 0.01%)

Knowing this classification can help you assess what to expect. However symptoms like a headache or nausea can also occur without treatment. In order to find out how often a treatment causes an adverse effect you need to know how many people who get the treatment have this problem and how many people without the treatment. If, for example, 10 out of 100 people (10%) who take a certain medication have a headache, and 5 out of 100 people (5%) who take a placebo, the medication has only caused headaches in 5 out of 100 people (5%). The remaining 5 out of 100 people (5%) would have a headache anyway.

Ideally, you would obviously like to know how often an adverse effect occurs in someone like you, for example in a person of your age, your sex and your state of health (including any other diseases or conditions). Unfortunately, information that is this detailed is often not available.

If a medication or vaccination has been approved, does that not automatically guarantee its safety?

Many people assume that such a guarantee exists, but there is no such thing as guaranteed safety. Medications and vaccinations undergo an extensive process before they are approved for use by state regulatory authorities like the European Medicines Agency (EMA) in Europe, or the German Federal Institute for Drugs and Medical Devices (BfArM), the Medicines and Healthcare products Regulatory Agency (MHRA) in the United Kingdom or the Food and Drug Administration (FDA) in the U.S. This approval process includes weighing up many aspects against one another (for example effectiveness, quality and safety). But regulatory authorities cannot take the same perspective as an individual patient.

Consumers have 

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a right to protection of their health, and the state takes a variety of measures to protect this right. German law forbids marketing what are called “questionable” medications (in German: bedenkliche Arzneimittel) or passing them on to others. On the other hand, people also have the right to make a personal decision and the right to accept risks for themselves. As a society we have decided in favor of having access to medications as fast as possible – even though this means that people who use new medications in the first few years after their approval carry a higher risk. Patients with a life-threatening disease, for example, surely would not like to hear that there is a medication that might help them, but that the government has decided to wait ten more years until more is known about its adverse effects.

Public interest naturally has top priority for regulatory authorities. But our society expects other aspects to be taken into account, too. In recent years, for example, public pressure to reduce the amount of animal testing has led to changes in testing procedures for medications. In addition, authorities also have to consider some manufacturers’ rights.

Regarding medications, regulatory authorities not only regulate what medications get on the market, but also have mechanisms for reacting to what happens after a medication has been introduced. This kind of system of long-term monitoring is called post-marketing surveillance. Together with all other procedures to detect adverse effects of medications and to react accordingly this is called pharmacovigilance.

If you suspect a medication has caused an adverse effect in you, you can tell your doctor or pharmacist about it. They can notify the competent authority. Patients can also call the phone number given on the package insert and inform the manufacturer of an adverse effect.

How does a drug approval process work?

Authorities like EMA, BfArM, MHRA and the FDA expect drug manufacturers to comply with some international as well as various national directives when developing medications. Regulatory authorities and drug manufacturers worked together to come up with these directives.

Manufacturers pay high fees to the regulatory authorities, which consult them while a medication is on its way to being approved. The manufacturers themselves can withdraw their applications for approval of their medications at any point during the process – or the authorities can refuse or limit approval of the medications, or withdraw approvals given in the past.

The regulatory authorities depend a lot on information given to them by the manufacturer. To what extent this information is available to the public differs from country to country. FDA, for example, publishes far more information on their homepage than other regulatory authorities generally do. EMA publishes some of the information they base their decisions on as well as information for patients. You can find this information here.

The tasks of the regulatory authorities are not limited to approving a drug or denying its approval, however. They can also determine whether (before or after a medication has been launched) more safety tests and measures are necessary. They also decide whether safety alerts are issued or whether these safety alerts have to be added to labels and product information for doctors and patients.

What does that mean in practical terms? For most medications that have bad results in safety tests during the development phase approval will never by applied for with the authorities. And most of the medications approval was applied for will be put into general use, too. According to EMA, for example, between 2006 and 2008 this authority handled applications for approval of 233 new drugs. 75 out of 100 applications (75%) were approved, and 8 out of 100 (8%) denied. In the remaining 17 out of 100 cases (17%), the manufacturers withdrew their applications for approval before the end of the process. This means that safety concerns might have been a problem in up to 1 out of 4 new medications.

How often are serious problems brought to light only after the launch of a medication?

This happens a lot. This is why from time to time there are media reports on adverse effects of medications. Researchers examined how drug approvals and more serious safety warnings in the U.S. that were needed later on relate to each other. To do this they assessed the number of black box warnings, the highest warning level the FDA issues. They get their name from a black border surrounding the text in order to catch the reader’s attention. The researchers found that between 1975 and 1999 about 8 out of 100 medications (8%) got a black-box safety warning within a few years of approval, and that about 3 out of 100 had to be withdrawn from the market (3%). Currently, about 1 to 2 medications are taken off the market each year for safety reasons.

However, the researchers also calculated that the risk probably increases to about 20% after 25 years. This means that eventually stronger warnings on safety issues have to be issued for 1 out of 5 new drugs after their approval, because these aspects were not detected in the approval process.

Court judgments also have an influence on whether a medication stays on the market and on the safety alerts to be provided by the manufacturer.

In Germany, BfArM is one of the institutions that collect and evaluate notifications of adverse effects. If German authorities suspect safety concerns about a certain drug they follow a step-by-step plan to analyze the risk and give the manufacturer the opportunity to respond. If the suspicion is not dispelled or the manufacturer cannot take any appropriate measures, the authorities can decide that safety notifications be added to the package insert, for example, that further trials be conducted, or even that approval be withdrawn.

As you look into this system more closely, you might find it shocking how frequently serious side effects are detected only after approval of a medication. But it is important not to forget how many lives drugs save and how often they can improve quality of life. This can be particularly important the first time an effective therapy for a certain disease becomes available. Using shortcut processes for risk and safety testing are less acceptable, on the other hand, if medications are already available that cause fewer or less severe adverse effects or that have a similar benefit.

How about non-drug therapies?

When talking about adverse effects the focus is often on the risks associated with an operation or a drug. This might be due to the fact that the risks of a surgical procedure are apparent to everybody and that medications, in contrast to medical devices and other types of therapy, are so strictly regulated and tested. This does not mean however that adverse effects are only an issue when it comes to medications and surgery.

Some people are so worried about side effects that they do not take their prescribed drugs or refuse to have a necessary operation – no matter what. They sometimes avoid “medicine” altogether and prefer using “natural remedies”. These are not harmless, either, however, mainly because illnesses left untreated or treated insufficiently can cause considerable harm. And if a therapy is not very effective (which is often the case if it is relatively “harmless” too), avoiding more effective treatment out of fear of adverse effects can have consequences that are a lot more severe than the therapy itself.

It is also possible that things you might not think about at all sometimes have adverse effects. Certain vitamins can be harmful if you take them at too high of a dose, for example. You can read more about this here. Other supposedly harmless interventions can also cause a surprisingly great amount of harm. For years parents were advised to turn their babies on their belly when laying them down, for example. But then it became clear that this increases the risk of sudden infant death syndrome. Another example can be found in the area of trauma relief: After a natural disaster or another traumatic event that affects many people, counseling teams to look after the victims are often sent out. Then, everyone who is affected is supposed to talk about what has happened and how they are feeling with someone who has received specific training. This method is called debriefing and is used although some people might not want to talk to the helpers, and research shows that such a general debriefing does more harm than good overall.

Avoiding truly unnecessary surgical or pharmaceutical therapy can of course lower your risk of harm. But not all harms can be avoided, and the fact that adverse effects can occur is usually not a good enough reason to not make use of the benefits medicine has to offer. It is crucial to get as much benefit and as little harm as possible.

How can I protect myself and my family from adverse effects?

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There are different ways of getting the best for your health and at the same time avoiding adverse effects as much as possible. Being well-informed is essential for weighing up the advantages and disadvantages of a particular treatment whenever you have the choice between various options. The following tips can help you do this:

  • If possible, the benefit, lack of benefit, and harm of a treatment should be presented in numbers. You can then evaluate how many people on average have a benefit from the treatment, how many people do not have a benefit and how many people are harmed.
  • The opposite can also be true, however: Some treatments have undeservedly gotten a bad reputation for adverse effects. The fact that two events occur at about the same time does not mean that one has caused the other. It may be a coincidence (and not an adverse effect) if you get a headache after taking a drug, for example.
  • Get enough information for you to be able to at least roughly understand the potential benefits and harms a therapy can have. You can then assess on your own how important the possible benefits are to you, what harms you consider acceptable, and weigh the benefits and harms against one another. The package insert also usually tells you about other risks when taking a certain drug (whether you are still able to drive safely, for example).
  • Carefully follow the advice when taking the medication or any other treatment – do not hesitate to ask so that you will know exactly what to do.
  • Your doctor or any other person who consults you in health issues needs to know if you have problems using the treatment or with adverse effects – particularly if you want to stop using the medication. Ending use of a drug that is important for your treatment without consulting a doctor can do more harm than adverse effects.
  • In case of an emergency brought on by a very severe adverse effect it is important to make an emergency call (in Germany: 112, in the United States: 911) and get help. You should be ready with information about the kind and amount of medications you have taken.
  • “More” does not always mean “better”: The benefit you can have does not necessarily depend on the dose – the therapy will surely achieve maximum benefit as prescribed. Adverse effects on the other hand generally do depend on the dose. Taking a higher dose might therefore not have any benefit for you at all – but the probability of adverse effects increases. The basic principle is: the ideal prescription is the one with the lowest dose that is proven to have the effect you need.
  • It may be important to take the medication for the length of time prescribed to prevent resistances to the drug from developing, and also to cure the illness completely. You can read more about this connection, using the example of antibiotics therapy, here.
  • Your doctor or pharmacist need to know what else you take in order to tell you as much as possible about potential interactions, which may be harmful for you and/or influence the effectiveness of the therapy. This also includes herbal remedies, vitamins or dietary supplements.
  • Doctors, dentists, pharmacists and other professionals dealing with your health should know whether you are allergic to a certain drug. You can write down this information in an allergy pass, for example, and take it along with you.
  • If you smoke you should think about quitting in order to lower the risk of some adverse effects. Trials have shown, for example, that it can lower the risk of complications if you do not smoke for one or two months before surgery. You can read more about this here.

When looking for patient information you should rather trust sources that are not of a promotional nature and that do not promise exaggerated benefits of the treatment. Other “information” might not mention side effects at all or play them down so that you are unable to make a balanced decision.

Now that you have read so much about adverse effects it might be important to remind yourself of the major advantages health care can have. We have put together the most important questions to help you make balanced decisions about your treatment here. To read more about what can help when taking several medications over a longer period of time, click here.

Weighing up the probability of benefit and harm: What helps people make decisions?

There is a risk of something harming us in everything we do. And we have to be able to handle uncertainties and risks in all areas of life. Medicine is no exception. Often the decision is easy and obvious, for example, if important advantages of the therapy far outweigh the rare disadvantages. But if the situation is not that simple, talking to your doctor or pharmacist might help you decide which advantages and disadvantages of the individual treatment options are most important to you.

Many people find it very important to understand what might happen and to know what they can do themselves in order to minimize harm for themselves. Reading health and patient information or using decision aids can help some people. A decision aid is formally structured information that aims to support you in weighing different facts against one another, if you have a hard time deciding between hoped for advantages and adverse effects.

When it comes to benefits and harms in health issues, people tend to be either overly optimistic or too worried. We are flooded with information on side effects, too. Yet sometimes we are not informed about the disadvantages of a therapy at all – and we are not always aware of this lack of information. Even though it is not easy to take the time to think about side effects, it is often worth it. We cannot make informed decisions for our health if we only know half the truth.

Author: Institute for Quality and Efficiency in Health Care (IQWiG)


  • IQWiG health information is based on research in the international literature. We identify the most scientifically reliable knowledge currently available, particularly so-called “systematic reviews”. These summarize and analyze the results of scientific research on the benefits and harms of treatments and other health care interventions. This helps medical professionals and people who are affected by the medical condition to weigh up the pros and cons. You can read more about systematic reviews and why these can provide the most trustworthy evidence about the state of knowledge here. The authors of the major systematic reviews on which our information is based are always approached to help us ensure the medical and scientific accuracy of our products.
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