Evidence

the longest trial behind each drug’s approval

every drug on this page was approved after trials measured in weeks, and most of them are taken for years. neither half of that sentence is a scandal on its own — short pivotal trials are how approval works, and long use is often the right clinical call. this page shows the single longest trial behind each approval, drug by drug, beside how many americans take it. it is the approval-package companion to our library-wide count of study horizons.

The two numbers to hold together.

Median antidepressant approval trial: 8 weeks. Median real-world duration of use in the United States: about five years [2]. And about one in four Americans taking an antidepressant this month has been taking one for ten years or more [4].

A different cut of the same question — every study horizon in our verified library, counted with denominators — lives at how long were these drugs actually tested?. This page is narrower: one number per drug, from the approval package alone.

The longest trial behind each approval

Weeks, from each drug’s approval package as quoted on its journey page [1]. The right column is people filling at least one US prescription in 2024 [5]. Sorted shortest trial first.

Where we cannot show you a number

Six drugs have no stated pivotal-trial duration in our record. A cleaner page would drop these rows; an honest one lists them with the reason, because absence here is a fact about the public record, not a finding about the drug.

  • sertraline (Zoloft) — our record carries no stated trial duration for the 1991 approval; the journey page explains the gap rather than guessing across it.
  • fluoxetine (Prozac) — same gap for the 1987 approval — no duration we can quote, so none appears.
  • gabapentin (Neurontin) — approved for seizures in 1993; its psychiatric use is off-label, so there is no psychiatric approval trial to time.
  • trazodone — a 1981 approval with a thin public record — the absence is in the record, not in our reading of it.
  • buspirone (BuSpar) — the current label format carries no study durations for the 1986 approval.
  • hydroxyzine (Vistaril) — approved in 1956, before efficacy trials were required at all — the oldest drug on this site predates the rules.

What this does and does not mean

Read the table in both directions. Short pivotal trials are not a scandal: a placebo-controlled trial of a few weeks is how the FDA establishes that a drug works at all, and it is the same standard for nearly every medicine you have ever taken. Some of these drugs also ran long trials — lamotrigine’s 78 weeks is a maintenance design testing relapse prevention, which is exactly the question long-term use raises.

What a six-week trial cannot tell you is what year six looks like: long-term side-effect rates, what stopping is like, whether the benefit persists. That evidence accumulates after approval, in studies with a patchier and less mandatory record — which is why each drug’s page on this site carries its post-approval research separately, and why the stopping evidence gets its own page. If you are weighing a prescription, the question this table hands you for your prescriber is short and fair: “the approval trials ran {n} weeks — what do we know about people who take it as long as I might?”

Common questions

How long are antidepressant clinical trials?
The pivotal trials behind FDA approvals typically run six to twelve weeks. A 2025 systematic analysis in The American Journal of Medicine put the median antidepressant trial at 8 weeks — against a median real-world duration of use of roughly five years.
Does a short trial mean the drug is unsafe?
No. Short placebo-controlled trials are the standard the FDA uses to establish that a drug works at all, and some drugs on this page also ran long maintenance trials — lamotrigine’s 78 weeks is one. What a short trial cannot tell you is what ten years on the drug does; that evidence comes from post-approval studies, which is a different and patchier record.
How many people take these drugs long term?
In the CDC’s national survey data, about one in four adults who took an antidepressant in the past month had been taking one for ten years or more. Overall, 13.2% of US adults used an antidepressant in the past 30 days in 2015–2018.
Why is there no number for sertraline or fluoxetine?
Because our label-sourced record carries no stated pivotal-trial duration for them, and we do not guess. Each drug’s journey page explains the specific gap. Absence of a number in our table is a fact about the record, not about the drug.
Where do the patient counts come from?
ClinCalc DrugStats, which publishes estimates from the federal Medical Expenditure Panel Survey. The figures are people filling at least one prescription in the United States in 2024, published with a roughly two-year lag, and they count prescriptions rather than outcomes.

Sources

Last verified 2026-08-31. Corrections change this date.

  1. Per-drug longest pivotal trial durations and approval years, read from each drug’s FDA-label-sourced approval journey on this site (APPROVAL_BASIS stage), retrieved from the production API 2026-08-31. Each drug name in the table links to the journey page carrying the label quote and SPL citation behind its number.
  2. Antidepressant Trial Duration Versus Duration of Real-World Use: A Systematic Analysis. The American Journal of Medicine (2025). Median trial duration 8 weeks; median real-world duration of use approximately 260 weeks. Preprint posted on medRxiv 2025-02-27; journal version cited. https://www.amjmed.com/article/S0002-9343(25)00286-4/abstract
  3. Brody DJ, Gu Q. Antidepressant Use Among Adults: United States, 2015–2018. NCHS Data Brief No. 377, September 2020. 13.2% of adults aged 18 and over used antidepressants in the past 30 days. https://www.cdc.gov/nchs/products/databriefs/db377.htm
  4. Pratt LA, Brody DJ, Gu Q. Antidepressant Use Among Persons Aged 12 and Over: United States, 2011–2014. NCHS Data Brief No. 283, August 2017. One-fourth of past-month antidepressant users reported ten or more years of use. https://www.cdc.gov/nchs/data/databriefs/db283.pdf
  5. ClinCalc DrugStats (Medical Expenditure Panel Survey, AHRQ), database version 2026.08, data year 2024, CC BY-SA 4.0. Patient counts as carried on each drug’s page on this site, transcribed 2026-08-31. https://clincalc.com/DrugStats/