📐 f2 Similarity Factor — Quick Concept & Beginner Guide
📘 Quick Concept
What is the f2 Similarity Factor?
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The f2 Formula — What It Measures
The f2 (similarity factor) is a logarithmic transformation of the mean squared difference between the Test and Reference dissolution profiles. It was developed by Moore & Flanner (1996) and is now the standard regulatory metric used by FDA, EMA and ICH for comparing dissolution profiles of solid oral dosage forms. A value between 50 and 100 means the two profiles are similar.
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f2 = 50 × log [100 / √(1 + Σ(Rt − Tt)² / n)]
Rt = mean % dissolved of Reference product at time point t
Tt = mean % dissolved of Test product at time point t
n = number of time points
When both products dissolve ≥ 85% at 15 min, f2 may not be required by regulation, but is still provided here.
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When Are Profiles "Similar"?
f2 ≥ 50: Profiles are similar — the Test product releases at essentially the same rate as the Reference.
f2 < 50: Profiles are NOT similar — the products release at meaningfully different rates.
FDA/EMA require ≥ 3 time points (excluding 0 min) for a valid regulatory f2 calculation.
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Regulatory Rules to Remember
• Only one time point should be above 85% dissolution for Test and Reference
• Minimum 3 time points excluding t=0
• Use mean % dissolved (not individual unit values) as input
• If ≥ 85% at 15 min for both — f2 may be waived per FDA/EMA guidance
• %CV at each time point ≤ 20% (early) or ≤ 10% (later) is recommended
💡 Beginner Tip
How to Use the Standalone f2 Calculator
- What to enter: Enter the mean % drug dissolved for each product at each time point. These are the averages across all units (e.g., 6 or 12 vessels), not individual readings.
- Time Points: Each row = one time point. Enter time in minutes. You can start with the default 7-row template (5, 10, 15, 20, 30, 45, 60 min) or customize completely.
- Test vs Reference: "Test (Tt)" = your generic or reformulated product. "Reference (Rt)" = the innovator / branded comparator product.
- Minimum rows required: At least 3 time points (excluding 0) are needed for a valid regulatory f2. Add as many as your SOP requires.
- 85% rule: If both products show ≥ 85% at 15 min, the tool will flag this — f2 may not be required in that case.
- Results: The tool shows the full step-by-step calculation, a graphical profile overlay, the f2 value, and a regulatory verdict.