The key is chance. If participants chose their group, or the researcher decided, the groups could differ in age, health or habits, and those differences would be confused with the effect of the treatment. Randomisation spreads those factors (even ones nobody has measured) evenly between groups.
The control is the other half: without a comparison group it is impossible to know how much of the observed change is due to the intervention and how much would have happened anyway through the passage of time, regression to the mean or the placebo effect.
The phase 3 trials that lead to a medicine's approval are almost always randomised and controlled. For most research peptides there are no trials of this kind, or only a few small ones.
Before giving weight to an efficacy claim, it is worth asking whether it comes from a randomised controlled trial, an observational study or an animal model. The design shapes everything else.
Related terms
Definition for educational and scientific purposes. It is not medical advice or a recommendation for use. NeoPeptidos products are sold labelled for research use only (RUO).