Impurities: Guideline for Residual Solvents
ICH Q3C(R9) recommends acceptable limits for residual solvents — the organic volatile chemicals used or produced in the manufacture of drug substances, excipients, and products — to protect patients from unnecessary exposure. It classifies solvents into three classes: Class 1 solvents to be avoided, Class 2 solvents to be limited by a Permitted Daily Exposure, and Class 3 solvents of low toxic potential. It provides two options for setting limits, a concentration-based Option 1 and a dose-based Option 2, and supplies the PDE values and the toxicological basis behind them.
What this does not cover
stated in the document's own scope- It does not address organic or inorganic impurities other than solvents; process and degradation impurities fall under ICH Q3A and Q3B.
- It does not cover elemental impurities such as heavy metals, which are governed by ICH Q3D and USP <232>/<233>.
- It does not govern mutagenic (DNA-reactive) impurities, which are controlled under ICH M7.
- It does not set the analytical validation criteria themselves, which are provided by ICH Q2(R2).
Always verify against the current published text before relying on it for a submission or inspection.
Overview
ICH Q3C(R9) is the ICH guideline governing residual solvents in pharmaceuticals. Solvents that are not fully removed during processing can remain in the drug substance, excipients, or finished product, and some carry meaningful toxicity. The guideline groups solvents into three classes by hazard, sets Permitted Daily Exposure values for the Class 2 solvents, and offers two ways to express a limit — a fixed concentration or a dose-based calculation. It also describes the toxicological method used to derive PDE values, so a manufacturer can calculate a limit for a solvent not already listed.
Legal basis & how it acquires force
Q3C(R9) is a harmonised ICH guideline that takes legal effect through regional adoption. The FDA implements it as guidance supporting the impurity and specification expectations of 21 CFR 211 and the new-drug application; the EMA adopts it as a CHMP scientific guideline; the residual-solvent limits are also carried into the pharmacopoeias, including the general chapters of the USP and Ph. Eur. Once adopted, applicants are expected to control residual solvents consistently with the classification and PDE values it sets.
Document structure
| Part | Covers |
|---|---|
| Introduction and scope | Purpose of limiting residual solvents and the products to which the guideline applies. |
| Classification of residual solvents | The Class 1 (avoid), Class 2 (limit), and Class 3 (low toxic potential) framework and the risk basis for each. |
| Limits of residual solvents | The Permitted Daily Exposure concept, Option 1 concentration limits, and Option 2 dose-based limits. |
| Analytical procedures | Expectations for methods used to quantify residual solvents, revised in R9 to consider solvent volatility. |
| Appendices and solvent tables | The PDE derivation method and the tabulated solvent lists with their class assignments and PDE values. |
Key requirements
- Classify every solvent used in or generated by the process as Class 1, Class 2, or Class 3 according to its toxicity.
- Avoid Class 1 solvents such as benzene and carbon tetrachloride wherever manufacture allows; justify and strictly limit any unavoidable use.
- Limit Class 2 solvents to their Permitted Daily Exposure, controlling them by either the Option 1 concentration limit or the Option 2 dose-based calculation.
- Treat Class 3 solvents as low-toxicity, with amounts up to 50 mg per day (0.5%) acceptable without further justification.
- Calculate PDE-based limits using the daily dose and, for Option 2, account for solvent contributions across all components of the product.
- Validate and apply analytical procedures capable of detecting and quantifying residual solvents at the relevant limits, considering solvent volatility.
- Report residual-solvent content and control it through the drug-substance and drug-product specification where warranted.
Implementation tips
- Map solvent use across the whole synthetic route and the excipients, since Option 2 sums contributions from every source in the finished product.
- Prefer Class 3 solvents in late synthetic steps where the chemistry allows, to simplify the control and testing burden downstream.
- Reserve routine residual-solvent testing for solvents actually used or carried forward, and support omissions with a documented risk rationale.
- Keep the headspace GC method and its system suitability aligned to the solvent volatilities that R9 highlights in its analytical-procedures section.
International alignment
Q3C sits in the ICH Q3 impurity family alongside Q3A (impurities in new drug substances), Q3B (impurities in new drug products), and Q3D (elemental impurities), and complements ICH M7 on mutagenic impurities. It is adopted by FDA, EMA, PMDA, and other ICH members, and its limits are reflected in USP and Ph. Eur. general chapters. R9, reached Step 4 on 24 January 2024, is a minor revision to the analytical-procedures section; the prior R8 dated from 22 April 2021.
ICH Q3C(R9): frequently asked questions
Quick answers to common questions about ICH Q3C(R9).
What are the three residual-solvent classes in ICH Q3C?
Class 1 solvents should be avoided because of their toxicity, Class 2 solvents should be limited to a Permitted Daily Exposure, and Class 3 solvents have low toxic potential and are acceptable up to 50 mg per day, equivalent to 0.5%.
What is the difference between Option 1 and Option 2 limits?
Option 1 sets a fixed concentration limit that assumes a maximum daily dose, while Option 2 derives an allowable concentration from the actual daily dose using the solvent's Permitted Daily Exposure, and can accommodate higher levels when the dose is small.
What changed in the R9 revision?
R9, adopted at Step 4 on 24 January 2024, is a minor revision to section 3.4 on analytical procedures, addressing the influence of solvent volatility; the classification and PDE values were not overhauled.
What is a Permitted Daily Exposure?
The PDE is the maximum acceptable intake of a residual solvent per day, derived toxicologically from a no-observed-effect level and modifying factors, and expressed in mg per day.