There are 2 questions here:
- Is not disclosing potential side effects during a clinical trial ethical?
- What does the "informed consent" of a clinical trial say about disclosing side effects?
According to Understanding Informed Consent
If you have given consent to participate in a clinical trial...you are
entitled to the following rights:
- To be told about all the risks, side effects, or discomforts that might be reasonably expected...
If you want to know details, such as any documented side effects of a
particular drug observed in earlier clinical trials, you must ask
for that information...
...which suggests that they might not tell you the exact side effects if you don't specifically ask them.
if you read a suggestion about informed consent by World Health Organization, they say that
Potential participants should be told if there are any known or
anticipated side effects...
So, it depends whom you ask and what someone believes about what is good and what not.
I think if in previous trials no deaths or other side effects with permanent consequences or other severe side effects have occurred, it may be acceptable to not disclose them - if the participants are informed that they may occur, they agree with this, they believe it's good that they participate in the trial and they are in peace with this.
Or, as said here: Informed consent in clinical research: Revisiting few concepts and areas (PubMed, 2013):
How much or up to what extent the information should be provided on various aspects of research, such as risks and benefits associated
with study intervention, is not clear and is rather a subjective
approach depending on the investigator. For example, recounting or
repetition of possible adverse effects of a study treatment may make
that treatment ill-advised when the treatment is not that risky. On
the contrary, if a lately appearing adverse effect is not disclosed to
the patient, it raises question on the validity of the consent as the
information was not provided adequately. Therefore, researchers are
recommended to provide the study-related information adequately,
judiciously, and truly maintaining an ethical balance between
expected risks and benefits of the intervention under investigation.