Selective serotonin reuptake inhibitors, commonly called SSRIs, are among the most frequently prescribed antidepressants. They are used to treat conditions such as depression and several anxiety-related disorders, although the specific medicine selected depends on the individual, diagnosis, previous treatment response, side effects, interactions, and other clinical considerations. Common SSRIs include fluoxetine, sertraline, paroxetine, citalopram, escitalopram, and fluvoxamine.
SSRI Conversion Calculator
What Is an SSRI?
An SSRI is a selective serotonin reuptake inhibitor, a class of antidepressant medication that affects serotonin signaling in the brain.
Examples include fluoxetine, sertraline, paroxetine, citalopram, and escitalopram. Fluvoxamine is another SSRI that is commonly associated with treatment of certain conditions, particularly obsessive-compulsive disorder.
SSRIs are widely used because they can be effective for depression and other mental health conditions, while their overall side-effect profile is often considered manageable compared with some older antidepressant classes. However, individual responses differ considerably. One person may respond well to one SSRI while experiencing troublesome side effects or inadequate symptom control with another.
This difference is one reason why medication switching sometimes becomes part of a treatment plan.
Why Compare SSRI Doses?
A healthcare professional may consider changing an SSRI for several reasons. These can include inadequate improvement, side effects, drug interactions, changes in treatment needs, or other clinical considerations.
However, an SSRI-to-SSRI comparison is not equivalent to saying that two medicines are clinically identical.
For example, if a person takes one SSRI at a particular dose, a calculated equivalent amount of another SSRI does not necessarily mean that the second medication should be prescribed at exactly that amount. The calculated number is simply an approximate reference comparison.
The NHS Specialist Pharmacy Service explains that switching between SSRIs can involve different strategies depending on the medications involved. For many SSRI-to-SSRI switches other than those involving fluoxetine, a direct switch may be possible under appropriate clinical supervision. Fluoxetine requires additional consideration because it and its active metabolite can remain in the body for an extended period.
Therefore, an equivalence calculator can be useful for understanding relative dose relationships, but it cannot replace individualized prescribing advice.
How to Use the SSRI Conversion Calculator
The calculator is designed to be straightforward.
Step 1: Select the Current SSRI
Choose the medication currently being considered from the Select SSRI field.
Available options are:
- Fluoxetine
- Sertraline
- Paroxetine
- Citalopram
- Escitalopram
- Fluvoxamine
Select the medication that corresponds to the current daily dose you want to compare.
Step 2: Enter the Current Daily Dose
Enter the current dose in mg/day.
For example, you might enter:
50 mg/day
The calculator accepts decimal values as well, allowing calculations such as 12.5 mg/day or 37.5 mg/day.
Step 3: Select the Target Medication
Choose the SSRI you want to compare against from the Convert To field.
For example, you could select:
Escitalopram
The calculator will then compare the entered medication and dose with the reference dose assigned to the selected target medication.
Step 4: Click Calculate
Press the Calculate button.
The calculator displays:
- Current medication
- Current daily dose
- Target medication
- Approximate equivalent dose
Step 5: Interpret the Result Carefully
The result should be treated as an educational estimate only.
It should not be interpreted as a personalized prescription or as instructions to take the calculated amount.
Antidepressant switching requires consideration of factors beyond milligram equivalence, including withdrawal symptoms, interactions, previous response, adverse effects, treatment indication, and the characteristics of the medicines involved.
SSRI Reference Doses Used by This Calculator
The calculator uses the following reference values:
| SSRI | Reference Dose Used |
|---|---|
| Fluoxetine | 20 mg/day |
| Sertraline | 50 mg/day |
| Paroxetine | 20 mg/day |
| Citalopram | 20 mg/day |
| Escitalopram | 10 mg/day |
| Fluvoxamine | 100 mg/day |
These values are used by the calculator as reference points for an educational mathematical comparison.
They should not be interpreted as universal therapeutic doses, maximum doses, starting doses, or recommended switching doses. Actual prescribing decisions can vary according to the patient, condition, age, other medicines, treatment history, and local prescribing information.
SSRI Conversion Formula
The calculator uses a proportional reference-dose formula.
The basic formula is:
Target Dose = Current Dose × Target Reference Dose ÷ Current Reference Dose
In mathematical notation:
Equivalent Dose = Current Dose × (Target Reference Dose / Current Reference Dose)
This means that the calculator first identifies the reference dose associated with the current medication. It then identifies the reference dose associated with the target medication.
The current dose is multiplied by the ratio between these two reference values.
Example of the Formula
Suppose a person enters:
- Current medication: Sertraline
- Current dose: 50 mg/day
- Target medication: Escitalopram
- Sertraline reference dose: 50 mg/day
- Escitalopram reference dose: 10 mg/day
The calculation is:
50 × (10 ÷ 50)
50 × 0.2 = 10 mg/day
Therefore, the calculator would display an approximate equivalent dose of:
10 mg/day of escitalopram
Again, this does not mean that someone taking 50 mg of sertraline should independently begin taking 10 mg of escitalopram. It only demonstrates the mathematical relationship used by this particular calculator.
Another SSRI Conversion Example
Consider a different example:
- Current medication: Fluoxetine
- Current dose: 40 mg/day
- Target medication: Sertraline
- Fluoxetine reference dose: 20 mg/day
- Sertraline reference dose: 50 mg/day
Using the formula:
40 × (50 ÷ 20)
40 × 2.5 = 100 mg/day
The calculator would therefore produce an approximate educational equivalence of 100 mg/day of sertraline.
This result illustrates why numerical equivalence should not automatically be interpreted as a switching schedule. The number is generated from the calculator's reference-dose ratio and does not account for individual clinical circumstances.
SSRI Conversion Examples Table
The following table demonstrates how the calculator's mathematical model works.
| Current SSRI | Current Dose | Target SSRI | Approximate Calculator Result |
|---|---|---|---|
| Fluoxetine | 20 mg/day | Sertraline | 50 mg/day |
| Fluoxetine | 20 mg/day | Escitalopram | 10 mg/day |
| Sertraline | 50 mg/day | Fluoxetine | 20 mg/day |
| Sertraline | 50 mg/day | Citalopram | 20 mg/day |
| Paroxetine | 20 mg/day | Escitalopram | 10 mg/day |
| Citalopram | 20 mg/day | Fluoxetine | 20 mg/day |
| Escitalopram | 10 mg/day | Sertraline | 50 mg/day |
| Fluvoxamine | 100 mg/day | Fluoxetine | 20 mg/day |
These are outputs of the calculator's reference-dose formula rather than individualized treatment recommendations.
Why SSRI Conversion Is Not a Simple 1:1 Process
One of the most important concepts when comparing antidepressants is that milligrams do not directly measure clinical effect across different medications.
A 20 mg dose of one medicine cannot automatically be treated as identical to a 20 mg dose of another.
Different SSRIs have different pharmacological characteristics. They may also have different half-lives, interaction profiles, approved uses, dosage forms, and tolerability patterns.
For example, fluoxetine has a comparatively long half-life and an active metabolite that can persist for an extended period. This can affect how clinicians plan switches involving fluoxetine. NHS Specialist Pharmacy Service guidance specifically highlights additional precautions when switching from fluoxetine.
Paroxetine is another medication for which withdrawal considerations can be particularly relevant. NHS guidance notes that withdrawal symptoms can occur after antidepressants are stopped and that some medicines, including paroxetine, can be more associated with withdrawal symptoms.
These factors demonstrate why an equivalence number is only one small part of a medication-switching assessment.
SSRI Switching and Withdrawal
Stopping an antidepressant suddenly can cause withdrawal symptoms. These may include dizziness, nausea, headaches, anxiety, sleep problems, sweating, unusual sensations, irritability, and other physical or psychological symptoms.
The likelihood and severity of withdrawal can vary between individuals and medicines.
The NHS recommends discussing discontinuation with a healthcare professional rather than stopping an antidepressant suddenly. Gradual dose reduction may be used to reduce withdrawal symptoms, and the pace can depend on the individual.
This is particularly important when someone is considering a medication change based only on an online conversion result.
What About Serotonin Syndrome?
Another important consideration during antidepressant switching is serotonin syndrome.
Serotonin syndrome is an uncommon but potentially serious reaction associated with excessive serotonergic activity. The risk can increase when serotonergic medicines are combined or when certain medicines are switched inappropriately. Symptoms can include agitation, confusion, sweating, tremor, changes in heart rate or blood pressure, increased body temperature, muscle abnormalities, and gastrointestinal symptoms.
This is one reason antidepressant changes should be planned by a qualified healthcare professional.
Do not use the calculator to decide whether two antidepressants can safely be taken together.
Factors That Can Affect an SSRI Switch
A clinician may consider several factors before changing antidepressants.
1. Current Medication
The medication being taken affects how a switch can be performed.
2. Target Medication
The characteristics of the new antidepressant also matter.
3. Current Dose
A person's current dose provides important information, but it does not independently determine the appropriate target dose.
4. Treatment Indication
The appropriate treatment approach may depend on whether the medicine is being used for depression, anxiety, obsessive-compulsive disorder, or another condition.
5. Previous Treatment Response
A previous response to an antidepressant can influence future treatment decisions.
6. Side Effects
If the reason for switching is a side effect, the prescriber may consider whether the proposed alternative has a different side-effect profile.
7. Other Medicines
Drug interactions are important. The FDA advises patients to tell their healthcare provider about prescription medicines, over-the-counter products, supplements, and herbal products because these can interact with antidepressants.
8. Withdrawal Risk
The duration of treatment, medication characteristics, dose, and individual circumstances can influence withdrawal risk.
Benefits of an SSRI Conversion Calculator
An educational SSRI conversion calculator can be useful for understanding how reference-dose comparisons work.
Quick Mathematical Comparison
Instead of manually calculating ratios, the tool produces the result automatically.
Multiple SSRIs
The calculator includes six commonly recognized SSRIs, allowing comparisons among the listed medications.
Easy Dose Entry
Users can enter a daily dose in milligrams, including decimal values.
Educational Understanding
Students, healthcare readers, researchers, and patients preparing questions for a clinician may find proportional dose calculations useful for understanding why different medications cannot always be compared on a simple milligram-for-milligram basis.
Useful Starting Point for Discussion
The result can help someone formulate questions for a doctor or pharmacist. It should not replace that conversation.
Limitations of the Calculator
The most important limitation is that this tool is based on approximate reference-dose relationships rather than a universally validated conversion standard.
The calculator does not account for:
- Individual patient characteristics
- Age
- Medical history
- Kidney or liver function
- Other medications
- Drug interactions
- Previous antidepressant response
- Previous withdrawal symptoms
- Treatment indication
- Duration of treatment
- Genetic differences in drug metabolism
- Specific switching strategy
- Tapering requirements
- Washout requirements
- Individual tolerability
Consequently, the calculated result should not be interpreted as a precise therapeutic equivalent.
Clinical guidance emphasizes that antidepressant switching strategies vary and should be selected according to the medications and individual circumstances involved.
Who Can Use This Calculator?
The tool can be useful as an educational resource for:
- People researching SSRI medications
- Students studying pharmacology
- Readers learning about antidepressant dose comparisons
- Patients preparing questions for their healthcare provider
- Caregivers seeking general educational information
- Writers and researchers reviewing approximate SSRI dose relationships
However, people currently taking prescription antidepressants should discuss medication changes with their prescriber or pharmacist.
Important Safety Information
Do not use an SSRI conversion result as instructions to change your medication.
Do not stop an SSRI suddenly unless a healthcare professional specifically instructs you to do so. Sudden discontinuation can cause withdrawal symptoms, and gradual reduction is often used when stopping antidepressants.
Do not combine SSRIs or change from one antidepressant to another based solely on a calculator result.
If you experience severe or concerning symptoms after changing antidepressant medication, contact a healthcare professional promptly. If you develop symptoms suggestive of a serious reaction, seek urgent medical attention.
If antidepressant treatment is associated with new or worsening suicidal thoughts or an urge to self-harm, seek immediate professional help or emergency assistance. NHS guidance recommends urgent assessment in such circumstances.
SSRI Conversion Calculator: Key Takeaways
The SSRI Conversion Calculator provides a simple mathematical estimate based on reference doses for six SSRIs.
The included reference values are:
- Fluoxetine: 20 mg/day
- Sertraline: 50 mg/day
- Paroxetine: 20 mg/day
- Citalopram: 20 mg/day
- Escitalopram: 10 mg/day
- Fluvoxamine: 100 mg/day
The calculator uses:
Target Dose = Current Dose × Target Reference Dose ÷ Current Reference Dose
This approach provides an approximate numerical comparison, but it does not represent a clinically validated conversion or switching schedule.
Actual SSRI treatment decisions are more complicated because antidepressants differ in pharmacology, interactions, half-life, withdrawal characteristics, and individual effects. Clinical guidance also shows that different SSRI switches can require different approaches.
For that reason, the best way to use this calculator is as an educational reference tool and as a way to understand approximate dose relationships—not as a substitute for individualized medical advice.
Frequently Asked Questions
1. What is an SSRI conversion calculator?
An SSRI conversion calculator is an educational tool that estimates an approximate equivalent dose between selected SSRIs using predefined reference doses. It does not provide a personalized prescription or validated switching schedule.
2. Which medications are included in this SSRI calculator?
The calculator includes fluoxetine, sertraline, paroxetine, citalopram, escitalopram, and fluvoxamine.
3. Is the SSRI conversion result medically accurate?
The result is an approximate mathematical estimate rather than a clinically validated dose conversion. Individual treatment decisions require professional assessment.
4. Can I switch SSRIs using the calculator?
No. The calculator should not be used as instructions for switching medications. SSRI switching strategies can differ depending on the specific medicines and the individual's circumstances.
5. Why aren't SSRI doses directly interchangeable?
Different SSRIs have different pharmacological properties, potency relationships, half-lives, interactions, and individual effects. Therefore, equal milligram doses do not necessarily produce equal clinical effects.
6. Can I stop my SSRI suddenly?
You should not stop an antidepressant suddenly without discussing it with a healthcare professional. Withdrawal symptoms can occur, and gradual dose reduction may be recommended.
7. Does the calculator include tapering schedules?
No. The calculator only estimates an approximate equivalent dose. It does not provide a tapering schedule, washout period, or cross-tapering plan.
8. Why is fluoxetine different when switching antidepressants?
Fluoxetine and its active metabolite have relatively long-lasting effects in the body. Specialist guidance therefore recommends additional consideration when switching from fluoxetine.
9. What is the difference between an equivalent dose and a prescribed dose?
An equivalent dose is an estimated numerical comparison based on a reference system. A prescribed dose is a clinical decision made after considering the patient's condition, treatment history, other medicines, risks, and expected benefits.
10. Can the calculator account for my other medications?
No. The calculator does not evaluate drug interactions. Your doctor or pharmacist should review all prescription medicines, over-the-counter medicines, supplements, and herbal products before a medication change.
11. Can two SSRIs be taken together?
Do not assume that two SSRIs can safely be taken together simply because they are included in this calculator. Combining serotonergic medicines can increase the risk of adverse effects, including serotonin syndrome in some circumstances.
12. Does the calculator work with decimal doses?
Yes. The calculator accepts decimal values, allowing users to enter doses such as 12.5 mg/day or 37.5 mg/day for mathematical comparison.
13. Is the result suitable for children?
The calculator is not designed to determine pediatric dosing. Children and adolescents require age- and condition-appropriate professional assessment and dosing guidance.
14. What should I do if I experience withdrawal symptoms?
Contact your healthcare professional for advice. Withdrawal symptoms can include dizziness, nausea, anxiety, sleep problems, headaches, unusual sensations, and other symptoms. The appropriate response depends on the medication and individual circumstances.
15. Who should I talk to before changing my SSRI?
Speak with your prescribing doctor, psychiatrist, or pharmacist before changing your antidepressant dose or switching medications. They can evaluate the reason for the change, your current treatment, other medicines, potential interactions, and an appropriate switching or tapering strategy.
Final Thoughts
The SSRI Conversion Calculator offers a convenient way to explore approximate dose relationships among fluoxetine, sertraline, paroxetine, citalopram, escitalopram, and fluvoxamine. Its calculation is based on a straightforward reference-dose ratio, making it useful for educational comparisons.
However, SSRI equivalence should never be treated as an exact science based solely on milligrams. A numerical result does not account for individual response, medication interactions, withdrawal risk, half-life, treatment indication, or the specific strategy needed to move from one antidepressant to another.
Use the calculator to understand the concept of approximate SSRI dose equivalence, and use the result as a starting point for questions—not as a medication-change instruction. Any actual adjustment, discontinuation, or switch should be planned with a qualified healthcare professional.
Medical disclaimer: This calculator and article are provided for general educational and informational purposes. The calculated equivalence is approximate and is not a clinically validated dose-conversion method or personalized medical recommendation. Never start, stop, combine, or change prescription antidepressants based solely on this calculator. Consult a qualified healthcare professional for individualized medication advice.