Most people know what side effects are. They are the results of something a person does, or of a medication that a person takes, or perhaps a decision a person makes; and these results are either unnecessary, or unwanted, or sometimes even counter to the effect the person was going for. For example, if a person takes Ibuprofen for a headache, they may become nauseated, which is certainly not an effect they want. In this same scenario, Ibuprofen could actually worsen their headache, as headaches are actually a common side effect of this medication. Ibuprofen is used for fever, but it can also induce fever. Many other drugs have similar side effect profiles.
But there is really no such thing as a side effect. There are only effects, which can be helpful, neutral, or harmful. Therefore the question becomes whether all the effects can be known, and whether the helpful effects are worth the cost of the harmful ones.
Now, it is impossible to predict with absolute certainty all the effects that will result from taking medicine. However, in the standard practice of medicine, it is said that all treatments have side effects. Any medical intervention, whether with pharmaceuticals, or surgery, or some other targeted treatment, will have unwanted, undesirable effects. They can be minimized, but not eliminated. All technology improves, as research proceeds, newer and better treatments will be found that are more effective, with less side effects. But fundamentally, at the philosophical level, side effects cannot be eliminated. This is because effects are seen as probabilities: there is a certain probability that the effect will be positive, and another (non-zero) probability that there will be a negative effect. Effects are seen this way because the individual cannot be fully known and understood. Any single person can only be understood as part of a particular group, and unless a treatment is known to affect all members of this group in the same way it is impossible to predict exactly how this individual will respond. Furthermore, since individuals necessarily differ from the group average there is always a chance that a single person will respond differently than the average group member. So even if the average person does not experience side effects it is possible for the individual to have unwanted results.
There is a different perspective. The philosophy outlined above assumes that medicine exists to act “on” the body, and that the results of this action can only be understood by aggregating the effects on a large group of people. A alternate view is that medicine works by acting “with” the body. Acting on the body is forcing the body to behave in a way which is against its natural inclination. Acting with the body is encouraging the body to respond in a way that it already wants to do. Taking this perspective means that side effects are not a necessary part of medical treatment, because if the body can be nudged in the right way there will be no unwanted results. This, however, requires attempting to understood each person as a unique individual, because the nudge needs to be slightly different for each person to account for their distinct nature.
Just as it is impossible to predict all effects of a treatment it is equally impossible to completely know and understand a unique human being. Fortunately, if a practitioner is treating “with” the body the risk of side effects is greatly reduced, because the body is being guided to respond in a way it already desires to do. This approach also understands that health is a process and not a destination. If the person is getting better overall, then even if something unpleasant happens it is easier to recognize that they are still moving in the right direction. Instead of simply trading one symptom for another each response by the body can be mapped onto the path towards health. An example might be a person receiving some form of treatment for their chronic digestive trouble. They stop having bloating and indigestion but then develop body aches and a fever. These new symptoms are not side effects of the treatment; rather, they are signs the body is moving in a positive direction, as the new symptoms show the body is becoming stronger and attempting to discharge the old disease. However, if the reverse is true, and treatment for body aches and fever result in indigestion and bloating, then even if the fever and aches go away the person has actually moved in the opposite direction, away from health. The disease has moved inwards.
In short, the perspective that takes side effects as a necessary evil is only one approach. This is highly useful in acute and emergency situations. Most people would rather be alive than dead, regardless of side effects. The other perspective is often more helpful in chronic conditions. In situations of long-term imbalance trading one symptom for another is unlikely to lead a person towards a state of greater health. Here it would be better to work with the body and maximize the innate healing potential we all carry inside us.