Celexa (citalopram) and Prozac (fluoxetine) belong to the same drug class, so a prescriber suggesting one instead of the other can sound like a minor change. Their FDA labels show real differences: who each is approved for, how long each stays in the body, which heart-rhythm limits apply, and which other medicines become harder to combine.
- Drug class: both are selective serotonin reuptake inhibitors (SSRIs). Yes, Celexa is an SSRI.
- Approved uses: Celexa covers major depressive disorder in adults only. Prozac covers depression, OCD, bulimia nervosa and panic disorder, and some uses extend to children.
- Half-life: about 35 hours for citalopram; fluoxetine and its active metabolite norfluoxetine linger for days to weeks.
- Heart rhythm: Celexa has a QT-based dose ceiling; Prozac carries a QT precaution without a matching ceiling.
- Interactions: fluoxetine is a potent CYP2D6 inhibitor; citalopram’s key interaction concern is CYP2C19 inhibitors and other QT-prolonging drugs.
Maybe the first SSRI did not help enough, or a pharmacist flagged an interaction. Either way, you are holding a prescription for one of these two medicines and wondering whether the change is meaningful. The short answer is that they work through the same mechanism but behave very differently once they are in your body, and those differences shape the decisions your prescriber makes.
Is Celexa an SSRI, and is Prozac one too?
Yes to both. The Celexa prescribing information on DailyMed describes it as “a selective serotonin reuptake inhibitor (SSRI)” indicated for major depressive disorder in adults. The Prozac label uses the same class wording. MedlinePlus puts it in plain terms: citalopram “is in a class of antidepressants called selective serotonin reuptake inhibitors (SSRIs)” and works by increasing the amount of serotonin in the brain, and its fluoxetine page places that drug in the same class.
Prozac is the older of the two; its label lists an initial U.S. approval in 1987, against 1998 for Celexa. Both share the class boxed warning about suicidal thoughts and behaviors in children, adolescents and young adults, and both list serotonin syndrome, bleeding risk, low sodium, angle-closure glaucoma and sexual side effects among their warnings. The differences sit in the details below.
The full comparison, label by label
| Feature | Celexa (citalopram) | Prozac (fluoxetine) |
|---|---|---|
| Class | SSRI | SSRI |
| Initial U.S. approval | 1998 | 1987 |
| FDA-labeled uses | Major depressive disorder in adults | Major depressive disorder (acute and maintenance); OCD (acute and maintenance); bulimia nervosa (binge-eating and vomiting behaviors); panic disorder, with or without agoraphobia |
| Combination uses | None | With olanzapine: depressive episodes of bipolar I disorder; treatment-resistant depression. Prozac alone is not indicated for either |
| Children and teens | Not approved for pediatric patients | MDD from age 8; OCD from age 7; with olanzapine for bipolar I depression from age 10. Not approved under age 7 |
| Half-life | Mean terminal half-life about 35 hours; steady state in about a week | Fluoxetine 1 to 3 days after a single dose and 4 to 6 days with chronic use; norfluoxetine 4 to 16 days |
| Heart rhythm | Warning: dose-dependent QTc prolongation; doses above 40 mg a day not recommended | Warning: postmarketing QT prolongation and torsades de pointes; use with caution if risk factors are present; no QT-based dose cap |
| Lower label ceiling | 20 mg a day over age 60, in hepatic impairment, in CYP2C19 poor metabolizers and with CYP2C19 inhibitors | Lower or less frequent dosing in cirrhosis; no age-based ceiling |
| Enzyme effects | Metabolized mainly by CYP3A4 and CYP2C19 | Potent CYP2D6 inhibitor; can make a normal metabolizer resemble a poor metabolizer |
| Contraindicated with | MAOIs (including linezolid and IV methylene blue), pimozide | MAOIs (including linezolid and IV methylene blue), pimozide, thioridazine |
| MAOI gap after stopping | At least 14 days before an MAOI | At least 5 weeks before an MAOI |
| Thioridazine gap after stopping | Not applicable | At least 5 weeks |
| Most common reaction (at least 5% and twice placebo) | Ejaculation disorder, mainly delay | A longer list, including nausea, insomnia, anxiety, nervousness, decreased appetite, tremor, yawning, rash, abnormal dreams and decreased libido |
| Stopping | Reduce gradually rather than abruptly whenever possible | Gradual reduction recommended; levels fall slowly on their own, which the label says may lessen discontinuation symptoms |
The dose figures above are label ceilings shown to explain why prescribers order ECGs or check medication lists. They are not a guide to anyone’s own treatment.
Half-life: why fluoxetine lingers
This is the difference people notice most, even if they never hear the word half-life. The Celexa label gives a mean terminal half-life of about 35 hours and says steady-state levels arrive within about a week of once-daily dosing. When citalopram is stopped, most of it is gone within days.
Fluoxetine is in a different category. The Prozac label states that fluoxetine itself has an elimination half-life of 1 to 3 days after a single dose and 4 to 6 days after chronic use, and that its active metabolite, norfluoxetine, has a half-life of 4 to 16 days. Because both substances are active and both clear slowly, they build up during regular use. The label adds that “even when dosing is stopped, active drug substance will persist in the body for weeks,” depending on the person, the previous dose and how long it was taken.
That property cuts both ways, and it explains several decisions a prescriber may make on your behalf:
- Dose changes show up slowly. The label warns that changes in dose “will not be fully reflected in plasma for several weeks.” A prescriber judging whether a dose works may wait longer with fluoxetine than with citalopram.
- Stopping can be gentler. The Prozac label notes that fluoxetine and norfluoxetine levels fall gradually after the last dose, which “may minimize the risk of discontinuation symptoms.” That does not make stopping risk-free; the label still recommends gradual reduction whenever possible.
- Switching away takes planning. Leftover fluoxetine can still interact with a new medicine for weeks. This is why the label requires at least 5 weeks before an MAOI antidepressant, compared with 14 days for Celexa, and why it says tricyclic antidepressant levels may need monitoring when fluoxetine “has been recently discontinued.”
Whether the long tail helps or complicates things depends on what comes next, so the route of any switch belongs to your prescriber. Our guide to changing or stopping antidepressants safely covers how a planned change is usually monitored.
Heart rhythm: a ceiling for one, a caution for the other
Both labels discuss QT prolongation, a change on the ECG linked with a dangerous rhythm called torsade de pointes. They treat it differently.
The Celexa label states that citalopram “causes dose-dependent QTc prolongation” and that torsade de pointes, ventricular tachycardia and sudden death have been seen in postmarketing reports. It recommends that Celexa not be given above 40 mg once daily, and sets a lower ceiling of 20 mg a day for people over 60, people with liver impairment, CYP2C19 poor metabolizers and people taking CYP2C19 inhibitors such as cimetidine. It advises avoiding citalopram in congenital long QT syndrome, bradycardia, low potassium or magnesium, a recent heart attack or uncompensated heart failure, and recommends stopping the drug if QTc measurements stay above 500 ms.
The Prozac label reports postmarketing cases of QT prolongation and ventricular arrhythmia, including torsades de pointes, and asks for caution in people with congenital long QT syndrome, a past history of QT prolongation, a family history of long QT or sudden cardiac death, and similar risk factors. It suggests considering an ECG before and during treatment in people with those risk factors. It does not set a QT-driven dose cap.
In practice, neither drug is free of a rhythm question. Citalopram simply has the firmer limit and the longer list of situations to avoid. Our article on citalopram and heart-rhythm risk goes deeper into who needs an ECG.
Fainting, palpitations, a known long QT, a family history of sudden cardiac death, low potassium from water pills or vomiting, liver disease, and any antiarrhythmic, antipsychotic or macrolide or fluoroquinolone antibiotic on your list. These change the risk with both medicines.
Interactions: where fluoxetine asks more questions
Here the two drugs diverge most for people on several medicines. The Prozac label calls fluoxetine “a potent inhibitor of CYP2D6,” a liver enzyme that processes many common drugs, and says it may make a person with normal enzyme activity “resemble a poor metabolizer.” It lists tricyclic antidepressants, many antipsychotics and antiarrhythmics such as propafenone and flecainide as drugs to approach with caution. In two studies described on the label, previously stable imipramine and desipramine levels rose more than 2- to 10-fold when fluoxetine was added, an effect that may persist for 3 weeks or longer after fluoxetine stops. The label also flags higher alprazolam levels, a longer diazepam half-life, and possible rises in phenytoin and carbamazepine levels.
Pimozide and thioridazine are contraindicated with Prozac because fluoxetine can raise their levels through CYP2D6 and because all three can prolong the QT interval. Thioridazine should not be started within 5 weeks of stopping Prozac.
Citalopram’s interaction profile centers on the QT question rather than on blocking enzymes. The Celexa label contraindicates pimozide, says to avoid other QT-prolonging drugs, and lowers the maximum dose to 20 mg a day alongside CYP2C19 inhibitors because those raise citalopram exposure. Both labels contraindicate MAOIs, warn about serotonin syndrome with other serotonergic drugs such as triptans, tramadol, lithium and St. John’s Wort, and warn about added bleeding risk with aspirin, NSAIDs and anticoagulants such as warfarin.
The practical point: if you take a heart-rhythm drug, an antipsychotic, a tricyclic or a seizure medicine, ask the pharmacist to rerun the interaction check before the first dose of the new SSRI, and again after the old one is stopped.
Who each label approves
For an adult with major depression, both labels apply. Beyond that, Prozac covers more ground. Its label includes obsessive-compulsive disorder, bulimia nervosa and panic disorder, while Celexa’s single indication is major depressive disorder in adults. Doctors do prescribe citalopram for anxiety and other conditions, but those uses are off label.
Age is the sharper divide. The Celexa label states it “is not approved for use in pediatric patients” and that two trials in 407 children and teens did not support pediatric use. The Prozac label reports pediatric efficacy for depression in ages 8 to 18 and for OCD in ages 7 to 17, and says safety and effectiveness have not been established below age 8 for depression or below age 7 for OCD. It also asks prescribers to monitor height and weight in children, because a 19-week trial found children on fluoxetine gained on average 1.1 cm less height and 1.1 kg less weight than those on placebo.
Older adults face the opposite asymmetry. Citalopram exposure rises with age (the label reports AUC 23% to 30% higher in people 60 and over), which is why the lower ceiling applies. Prozac has no age-based ceiling.
Whichever drug you take, improvement is gradual. The Prozac label notes that the full effect may be delayed until 5 weeks of treatment or longer. Our article on how long antidepressants take to work sets out what to expect week by week, and if you are comparing citalopram with another common option, see Celexa vs. Zoloft.
Common misconceptions
“Same class means the same drug.” Both block serotonin reuptake, but the labels differ on approved uses, ages, half-life, heart-rhythm limits and interactions. A switch is a new set of rules.
“Prozac is approved for PMDD.” MedlinePlus lists premenstrual dysphoric disorder among fluoxetine’s uses, and that use was marketed under the discontinued brand Sarafem. The current Prozac label reviewed for this article does not list PMDD among its indications.
“Because fluoxetine lingers, I can stop it whenever I like.” The slow decline may soften discontinuation symptoms, but the label still recommends gradual reduction and lists symptoms reported on stopping. MedlinePlus is plain: do not stop fluoxetine without talking to your doctor.
“Celexa is the heart-risk drug; Prozac is heart-safe.” Both labels carry a QT warning. Citalopram has a hard dose ceiling; fluoxetine has caution language and contraindications with two QT-prolonging drugs.
“Once I switch, the old drug is out of the picture.” After fluoxetine, active drug can remain for weeks, which is why the label sets 5-week gaps before an MAOI or thioridazine. Changing or stopping either medicine is a decision to make with the prescriber who knows your history.
Sources
- DailyMed: Celexa (citalopram) prescribing information
- DailyMed: Prozac (fluoxetine) prescribing information
- MedlinePlus: Citalopram
- MedlinePlus: Fluoxetine
Evidence checked November 2, 2026. General information only; do not start, stop or change an antidepressant without your prescriber. In a crisis in the U.S., call or text 988.