Editor's note: This piece discusses mental health issues. If you have experienced suicidal thoughts or have lost someone to suicide and want to seek help, you can contact the Crisis Text Line by texting "START" to 741-741 or call the Suicide Prevention Lifeline at 800-273-8255.

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Two people can take the same antidepressant at the same dose and have very different experiences. One may improve within weeks, while another may develop side effects or feel little benefit. This variation is well documented and is one reason clinicians increasingly look at biology, not just symptoms, when treatment is not going as expected. Large treatment studies have also shown that only about one-third of patients achieve remission after a first antidepressant trial — which helps explain why medication selection often involves careful adjustment rather than a one-size-fits-all approach.