Fluoxetine (Prozac): uses, how long it takes to work and side effects

Fluoxetine is the generic name for Prozac. It is an antidepressant in the selective serotonin reuptake inhibitor (SSRI) class, the same class as sertraline (Zoloft) and escitalopram (Lexapro). It comes as capsules, tablets and liquid. It is usually taken once a day in the morning (sometimes twice, in the morning and at noon); the delayed-release capsule is taken once a week.

It is not a controlled substance and is not habit-forming like sedatives or sleeping pills. It still needs a prescription and medical follow-up.

What sets it apart is its long half-life. Per the FDA label, fluoxetine takes 4 to 6 days to fall by half with daily use, and its active metabolite, norfluoxetine, 4 to 16 days. That is why it takes about 4 to 5 weeks to reach a steady level in the blood, and stays in the body for several weeks after the last dose.

According to the FDA-approved Prozac label, fluoxetine is indicated for:

According to MedlinePlus, it can take 4 to 5 weeks or longer before you feel the full benefit. Some people first notice better sleep or appetite, and mood improves later.

Per the FDA label, for depression in adults the usual starting dose is 20 mg a day in the morning; in children, 10 or 20 mg. The doctor decides the dose for your case.

Keep taking it even if you do not notice a change yet, and do not raise or lower the dose on your own. If you see no improvement after several weeks, or side effects bother you, talk with the doctor: she can adjust the dose or consider another option.

According to the Prozac Medication Guide and MedlinePlus, the most common side effects are:

Prozac, like all antidepressants, carries the FDA's strongest warning (the boxed warning): antidepressants increased the risk of suicidal thoughts and behaviors in children, teens and young adults. MedlinePlus describes the higher risk as up to age 24.

The risk is highest in the first months and when the dose changes. Patients and families should watch for changes in mood or behavior: more sadness, anxiety, irritability, aggression, impulsiveness or talk of dying. If that happens, call the doctor right away; if there is danger, use the crisis lines below.

The doctor sees patients of all ages; patients under 21 are seen with a parent or legal guardian on camera for the whole visit.

This is an uncommon but serious reaction that can happen when fluoxetine is combined with other medicines that raise serotonin (other antidepressants, triptans for migraine, tramadol, lithium, tryptophan or St. John's wort). Per the Medication Guide, get help right away if you have:

Per the FDA label, fluoxetine must not be combined with:

• MAOI antidepressants (such as phenelzine or tranylcypromine), or with linezolid or IV methylene blue. Wait 14 days after stopping an MAOI before starting fluoxetine, and at least 5 weeks after stopping fluoxetine before starting an MAOI, because of its long half-life.
• Pimozide and thioridazine, because they prolong the heart's QT interval. Thioridazine also requires a 5-week wait after stopping fluoxetine.

Heart rhythm (QT). The label reports cases of QT prolongation and arrhythmias. It is used with care in people with rhythm problems, low potassium or magnesium, or who take other medicines that prolong the QT.

Bleeding. Taken with aspirin, ibuprofen, naproxen or blood thinners such as warfarin, it raises the risk of bleeding.

Tell the doctor about every medicine, vitamin and natural product you use.

If you miss a dose, take it when you remember, unless it is almost time for the next one; then skip it. Do not take a double dose (MedlinePlus).

Do not stop suddenly without talking to the doctor. Stopping an antidepressant abruptly can cause dizziness, irritability, anxiety, "brain zap" sensations or sleep problems. Because fluoxetine leaves the body very slowly, its level falls gradually even after the last dose; still, any dose change is made as your doctor directs.

If you are pregnant, planning a pregnancy or breastfeeding, decide with the doctor. Per the label, SSRIs used later in pregnancy may raise the risk of persistent pulmonary hypertension in the newborn, and some babies exposed late in the third trimester have had complications. But untreated depression has risks too, and the label notes that women who stop their antidepressant during pregnancy relapse more often. Do not stop on your own: talk with the doctor first.

Both are SSRIs and work in a similar way. The main differences, per their labels:

• Approved uses: fluoxetine covers bulimia and depression in children from age 8; sertraline covers PTSD, social anxiety disorder and premenstrual dysphoric disorder.
• Half-life: fluoxetine's is much longer, so its effect and its interactions last for weeks after stopping.
• Neither is "better" for everyone: the doctor chooses based on your symptoms, age, other medicines and how you have done before.