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The placebo effect is a known complication of trial interpretation.

PLACEBO IN CLINICAL TRIALS
The placebo, a pharmaceutically inert substance (typically a sugar pill), is the clinical researcher's analogue to the scientist's control experiment. To prove a new treatment effective above and beyond the psychological results of a simple belief in the ability of the drug to cure, a researcher compares the results of the experimental treatment for an illness with those obtained from the placebo. The placebo-controlled trial “is widely regarded as the gold standard for testing the efficacy of new treatments.”[17]

Interest in placebo effects began only with the widespread adoption of the placebo-controlled clinical trials after World War II. The randomized clinical trial was a major methodological breakthrough in medicine and the best evidence for new treatment came from randomized placebo-controlled (RCT) double-blind studies. It was noticed that patients improved, sometimes dramatically, in placebo control arms. Henry Beecher popularized this observation in his famous proto-meta-analysis which claimed that about 35% of the patients responded positively to placebo treatment.[18,19]

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ETHICS OF PLACEBO-CONTROLLED TRIALS
The use of a placebo in clinical research continues to be a topic of debate in the medical community in recent times. Some argue that the use of placebos is often unethical because alternative study designs would produce similar results with less risk to individual research participants. Others argue that the use of placebos is essential to protect the society from the harm that could result from the widespread use of ineffective medical treatments.

Critics of placebo-controlled trial or trials that include an untreated control group cite Article 11.3 of the Declaration of Helsinki: “In any medical study, every patient including those of control group, if any should be assured of the best proven diagnostic and therapeutic methods and no patient should suffer from unnecessary pain.”[20]

In randomized clinical trials, for conditions having no effective treatment, the control regimen with which the new treatment is compared, is warranted to establish the evidence. However, when an effective treatment already exists, it is unethical to create a placebo group that will receive no treatment. In other words, patients are deprived from an already existing effective therapy. The objective of testing such drugs to establish whether the new drug is better in efficacy or safety when compared to the existing drug/s placebo controlled trial considered unethical.

The association of placebo effects with RCTs has caused confusion because the response in the placebo arm is not necessarily a genuine psychosocial response to the simulation of treatment. In fact, the observed response to placebo in RCTs may reflect the natural course of the disease, fluctuations in symptoms, regression to the mean, response bias with respect to the patient's reporting of subjective symptoms and other concurrent treatments.[3,4]
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Ethics of placebo in children
The use of placebo in children is more restricted than in adults, because children cannot consent. Placebo should not be used when it means withholding effective treatment, particularly for serious and life-threatening conditions. The use of placebo is often needed for scientific reasons, including pediatric trials. The use of placebo may be warranted in children as in adults when evidence for any particular treatment is lacking or when the placebo effect is known to be very variable (e.g., pain, hay fever). As the level of evidence in favor of an effective treatment increases, the ethical justification for the use of placebo decreases.[24]

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3601706/