A clinical trial is a research study in people that tests whether a treatment is safe and whether it works, following a written plan reviewed by an ethics committee.
Trials usually run in phases. Early phases look at safety and dose in small groups. Later phases compare the treatment with a placebo or an existing treatment in larger groups.
Most of what is known about ADHD medication comes from clinical trials. Many earlier ADHD studies recruited mostly boys, which limits what they can tell us about women and about adults diagnosed later in life.
Trials follow a protocol, are usually registered in advance and report side effects as well as benefits. That's what makes their results more trustworthy than one person's account.