Why is a placebo group included in clinical trials?

A placebo is a treatment (tablet, injection, sham surgery or other procedure) that appears real but is not. Theoretically, a placebo has no known therapeutic benefit on a patient’s health. However, placebos are still a very important part of clinical trials, as they are often used to ensure the accuracy of clinical trial results. 

It is common for trial participants to feel legitimate changes purely because they know they took/had a treatment, even if the treatment was inactive. This is known as the ‘placebo effect. It is believed that around one in three people feel the placebo effect.

Placebo groups in clinical trials

When conducting clinical trials, researchers want to be as thorough as they possibly can be, leaving nothing to chance. Quite simply, they need to know if any supposed benefits of a treatment used in a clinical trial are real and are not due to the placebo effect.

There are many different factors that influence the placebo effect. For example, a participant  in a study who already believes in the new treatment before receiving it may have a natural bias that leads to the placebo effect.

Whilst the placebo effect usually doesn’t last for the length of a full clinical trial study, it is nonetheless essential for researchers to distinguish between the placebos and ‘actual’ responses. This way, they are one step closer to establishing whether a treatment will have positive effects on the general population.

Why are placebo groups randomly allocated?

Before we talk about the reason for random allocation of placebo groups here are some important definitions:  

Blinding refers to the concealment of group allocation from one or more individuals involved in a clinical research study, most commonly a randomized controlled trial

Randomisation  “In research, the process by which participants in clinical trials are assigned by chance to separate groups that are given different treatments or other interventions. Neither the researcher nor the participant chooses which treatment or intervention the participant will receive. Using chance to assign people to groups means that the effects of the treatment or intervention that participants receive can be compared more fairly.” (source https://www.cancer.gov/publications/dictionaries/cancer-terms/def/randomization

Double-blind study “A type of clinical trial in which neither the participants nor the researcher knows which treatment or intervention participants are receiving until the clinical trial is over. This makes results of the study less likely to be biased. This means that the results are less likely to be affected by factors that are not related to the treatment or intervention being tested.” (source https://www.cancer.gov/publications/dictionaries/cancer-terms/def/randomization

What is the difference between blinding and randomization?

Randomisation can be used to prevent selection bias and confounding factors in a trial. Whereas, blinding reduces the opportunity for bias to be introduced after the interventions have been allocated

Read More about randomisation & blinding here 

Placebo groups are randomly allocated to maintain the integrity of a trial. This group is known as the ‘control group’. In many clinical trials, participants don’t know whether they’re being given a placebo or not. Additionally, even the doctors running the trial do not always know who is getting the placebo. When neither the doctor or patient know if they are in the treatment or placebo group it is known as a double-blind clinical trial. This ensures that there will be less or no unconscious bias that influences the results. The more random the allocation of placebo groups, the more reliable the results. 

Double-blind randomisation also ensures there can be no selection bias on the part of doctors, clinicians or researchers, towards patients that they feel may have a greater need for the trial treatment. What types of trials typically have placebos?

Placebos are generally not used in cancer clinical trials as placebos in cancer care could be detrimental to the participant if denied real treatment. 

Placebos are often used as a control in conditions where no proven effective therapy exists. The use of placebo may raise ethical considerations when there are available, effective treatments for a disease because it may be seen as denying patients treatment. However, they are still arguably ethically justifiable when participants are not exposed to extreme risk.  The placebo arm helps researchers determine if the treatment being investigated has an impact on treating the condition. For example, mood fluctuations are a symptom of depression. It is important to distinguish between the actual effects of an antidepressant being tested in a trial and the perception of being given a treatment. 

Why do some clinical trials not have placebo groups?

Studies investigating proven treatments, such as  Phase 4 clinical trials, which serve the purpose of monitoring the effectiveness of new treatments after approval and collecting information about any adverse effects associated with long-term and widespread use or where approved treatments are compared to each other to determine which treatment is superior,  the placebo effect is not relevant.

Sometimes a combination therapy might be more effective than an existing treatment, in which a new trial would be initiated, and the trial parameters could include a control group and a placebo group again.  

As mentioned earlier placebos are considered unethical in patients with life-threatening cancer or other illnesses. In these disease cases the chance of a placebo is neither good for the patient nor the trialist and also breaches the standard of care that each patient deserves. 

The ‘Nocebo’ effect

The nocebo effect refers to the opposite of the placebo effect. This occurs when a patient is given a treatment, and they have a negative response because they were expecting it. It usually wears off quickly, but it is of course another consideration for those conducting trials

White Coats Foundation 

White Coats is a Not for Profit Australian-based charity. The Foundation was established in recognition of the need to raise awareness about the role of clinical trials in advancing medical science and healthcare. We are providing information about clinical trials through our Webinar Series and Our Blogs.

At White Coats Foundation, we provide access to credible resources and information to help guide people’s journey in understanding clinical trials and consumer and community involvement in research.

Please note: White Coats blogs are informational only and do not constitute advice. Please contact your relevant healthcare professional for advice on clinical trials for you.

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