A randomised controlled trial is a study where participants are assigned by chance to receive either the treatment being tested or a comparison, such as a placebo or usual care.
Random assignment means the groups should be similar in everything except the treatment. Any difference in outcome can then be attributed to the treatment rather than to who chose to take it.
RCTs are the strongest single type of study for testing whether a treatment works. They are less suited to questions about long-term outcomes, rare side effects or people who were excluded from the trial.
That last point matters for ADHD and autism. Many trials exclude people with co-occurring conditions, and that describes a large share of the adults who come for assessment. A treatment that worked in a trial still needs to be tried carefully with you.