By the PharmaTradz BD Team. Published October 2026.
Water for injection (WFI) is the highest grade of bulk pharmaceutical water. It is used to make injections and other parenteral products, and it must meet a strict limit for bacterial endotoxins (fever-causing fragments of bacteria) as well as tight chemical and microbial limits. Purified water (PW) is the grade used for tablets, oral liquids, creams and most non-sterile products. It has similar chemical limits but a looser microbial limit and, in general, no endotoxin test. Both are defined in the European Pharmacopoeia (Ph. Eur.) and the United States Pharmacopeia (USP).
Water is the most used raw material in most pharmaceutical plants, and it is made on site, every day, in a system the buyer never sees. That makes the water system one of the first things a good auditor walks.
The pharmacopoeial water grades
Purified Water (Ph. Eur. monograph 0008; USP monograph Purified Water) is made from drinking water by any suitable method, such as reverse osmosis, ion exchange or distillation. It is the default grade for non-sterile medicines and for most steps in making APIs.
Water for Injections (Ph. Eur. monograph 0169; called Water for Injection in the USP) is made by distillation or by a process equivalent to distillation. On top of the purified water tests, it must contain less than 0.25 endotoxin units per mL and meet a microbial limit a thousand times tighter.
Highly Purified Water (HPW) was a third Ph. Eur. grade (monograph 1927). It had the same quality as WFI but could be made by non-distillation methods, so it could not be called WFI. Once the WFI monograph itself allowed non-distillation methods, HPW became redundant. The monograph was suppressed in 2019, and Ph. Eur. 9.6 was the last edition to contain it. You may still see "HPW" on older system documents.
Sterile Water for Injection is a finished product, not a utility. In the USP it is Water for Injection that has been sterilised and packed in single-dose glass or plastic containers of not more than 1 litre, with nothing added. The Ph. Eur. equivalent, sterilised water for injections, is described in the same 0169 monograph. Other packaged grades include Bacteriostatic Water for Injection (with an added antimicrobial agent), Sterile Water for Irrigation and Sterile Water for Inhalation. These are registered medicines with their own labels, and they are bought and sold like any other product.
Key specifications compared
The table below summarises the main limits for the bulk grades. Conductivity measures dissolved ions (salts); TOC (total organic carbon) measures organic contamination; CFU means colony-forming units, the count of live bacteria grown from a sample.
| Test | Purified Water | Water for Injection(s) |
|---|---|---|
| Conductivity | Ph. Eur.: not more than 4.3 µS/cm at 20 °C. USP: three-stage test in chapter <645>, stage 1 limit 1.3 µS/cm at 25 °C | Ph. Eur. and USP: the same three-stage test, stage 1 limit 1.1 µS/cm at 20 °C or 1.3 µS/cm at 25 °C, stage 2 limit 2.1 µS/cm |
| TOC | Not more than 0.5 mg/L (500 ppb) | Not more than 0.5 mg/L (500 ppb) |
| Bacterial endotoxins | Not required in general. Ph. Eur. sets less than 0.25 IU/mL only for water used to make dialysis solutions | Less than 0.25 IU/mL (Ph. Eur.) or 0.25 USP Endotoxin Units/mL |
| Microbial action level | 100 CFU/mL | 10 CFU/100 mL |
| Other | Ph. Eur. nitrates not more than 0.2 ppm | Ph. Eur. nitrates not more than 0.2 ppm |
Two points often confuse people. First, the chemical limits for the two grades are close, so you cannot tell them apart from a conductivity reading alone. The real differences are endotoxins, microbial quality and how the water is made and held. Second, the microbial figures are action levels, not pass or fail limits for each sample. In the Ph. Eur. they sit in the production section of the monographs; in the USP they appear in the general information chapter <1231> Water for Pharmaceutical Purposes. Plants set their own tighter alert levels from their qualification data.
How each grade is produced
Every system starts with drinking water (potable water) meeting national or WHO standards. A pre-treatment train then removes particles, hardness and chlorine, usually with media filters, water softeners and activated carbon or a chemical dosing step.
Purified water is then usually made by reverse osmosis (RO), where water is pushed through a membrane that holds back most salts, organics and bacteria, often in two passes. This is commonly followed by electrodeionisation (EDI), which uses an electric field and ion-exchange resins to remove the remaining ions without chemical regeneration. Ultraviolet (UV) lamps and filters may follow.
WFI was traditionally made only by distillation: water is boiled and the steam condensed, leaving salts, bacteria and endotoxins behind. Multiple-effect and vapour-compression stills are the common designs. Since April 2017 (Ph. Eur. Supplement 9.1), the revised Ph. Eur. WFI monograph has also allowed a purification process equivalent to distillation, such as single- or double-pass RO combined with techniques like EDI, ultrafiltration or nanofiltration. The USP has long allowed distillation or a process "equivalent or superior to distillation". In practice, a membrane-based WFI system needs especially careful design and monitoring, because membranes run cold and are not an absolute barrier to bacteria.
Storage and distribution: loops, temperature and sanitisation
Making good water is only half the job. Water that sits still grows bacteria, which form a slimy layer called biofilm on pipe walls and shed endotoxins. So pharmaceutical water is stored in a tank and pumped continuously around a closed distribution loop, with valves at each point of use.
WFI is usually kept in a hot loop. EU GMP Annex 1 (2022) gives "constant circulation at a temperature above 70°C" as an example of how to minimise microbial growth, and the WHO guideline uses the same figure. Where a process needs cool water, heat exchangers at the point of use cool it down just before use. Purified water loops often run at ambient temperature and are sanitised on a schedule, using hot water, ozone, chemical agents such as hydrogen peroxide or peracetic acid, or UV. Whatever the method, it must be validated, and chemical agents must be shown to be rinsed out.
Good design means polished stainless-steel pipework that slopes so it can drain fully, fast (turbulent) flow, sanitary valves and no dead legs, which are stagnant branches where water does not circulate.
Where each grade is used
The EMA guideline sets out the minimum grade for each use. In brief:
- Injections and other parenterals: WFI, including the final rinse of equipment and containers.
- Tablets, capsules, oral liquids and other non-sterile products: purified water.
- Eye drops and sterile nasal or ear products: at least purified water, with a higher grade where risk assessment calls for it.
- Nebuliser solutions: purified water, or WFI where the product must be sterile and non-pyrogenic.
- APIs: drinking water can be used for early synthesis steps, purified water is generally expected for final isolation and purification, and WFI for sterile, pyrogen-free APIs.
- Cleaning: the final rinse should use water of at least the grade used in the product.
The rules that apply
In Europe, the EMA's Guideline on the quality of water for pharmaceutical use (EMA/CHMP/CVMP/QWP/496873/2018) was published in July 2020 and came into effect on 1 February 2021. It replaced the earlier note for guidance on water quality and reflects the revised WFI monograph and the removal of HPW. On the GMP side, EU GMP Annex 1 (Manufacture of Sterile Medicinal Products, in force since 25 August 2023) has a section on water systems. It expects WFI systems to have continuous TOC and conductivity monitoring, alert levels based on qualification data, and an investigation of every action-level excursion.
The WHO's Good manufacturing practices: water for pharmaceutical use (TRS 1033, Annex 3, 2021) is widely used by inspectorates in many markets. It describes a three-phase qualification covering at least 12 months, so that seasonal changes in feed water are captured. In the US, the FDA's Guide to Inspections of High Purity Water Systems (1993) remains a key reference for investigators, alongside USP <1231>.
A worked example
Suppose you are auditing a contract manufacturer that makes both an oral syrup and an injection. The weekly WFI sample from one point of use shows 14 CFU per 100 mL, above the action level of 10. The plant's alert level is 5. A well-run site would raise a deviation, resample, check sanitisation and maintenance records for that outlet, identify the organism, and assess every batch made with water from that loop since the last good result. It would then decide whether a sanitisation, a hardware fix or a corrective action is needed. If the same outlet also feeds the final rinse in the syrup suite, the review must cover that use too. An auditor who finds the result filed without investigation has found a real weakness.
What buyers and auditors should check
Ask for a simple schematic of the water system, from incoming drinking water to each point of use, and check it matches what you see on the floor. Ask which grade is used for your product and for the final equipment rinse, and whether WFI is made by distillation or by membranes.
Review the qualification report, including the 12-month performance phase, and confirm that later changes went through change control and requalification. Our guide to IQ, OQ and PQ explains what a qualification package should contain.
Look at trend data for the last 12 months: microbial counts, endotoxins, TOC and conductivity per sampling point. Check that alert and action levels are defined and that excursions led to investigations, treated with the same discipline as an OOS result. Check the sampling plan: every point of use should be sampled on a rotation, and samples should be taken the way production draws water, through the same hoses.
Ask about sanitisation: method, frequency, records and validation. For a hot WFI loop, look at the temperature chart and ask what happens if the loop drops below its set point. Finally, ask whether online TOC and conductivity instruments are calibrated, and whether their alarms are reviewed and acted on.
How PharmaTradz can help
Our page on sterile injectable contract manufacturing explains what to verify in a WFI-dependent site, and our oral liquid contract manufacturing page covers purified-water products. Tell us the product, dosage form and market, and send us an RFQ. For other terms, see our pharma abbreviations A-Z.
Frequently Asked Questions(FAQs)
What is the difference between water for injection and purified water?
Both have similar chemical limits for conductivity and TOC, but water for injection must also contain less than 0.25 endotoxin units per mL and meet a much tighter microbial action level of 10 CFU per 100 mL, against 100 CFU per mL for purified water. WFI is used for injections; purified water for non-sterile products.
How is water for injection produced?
Traditionally by distillation. Since April 2017 the Ph. Eur. has also allowed a process equivalent to distillation, such as reverse osmosis combined with electrodeionisation or ultrafiltration, and the USP allows distillation or a process equivalent or superior to it.
What is the endotoxin limit for WFI?
Less than 0.25 endotoxin units per mL in both the Ph. Eur. (IU/mL) and the USP (USP Endotoxin Units/mL).
Is sterile water for injection the same as WFI?
No. WFI is a bulk utility made and used inside a factory, while Sterile Water for Injection is WFI that has been sterilised and packed in sealed single-dose containers, sold as a finished product.
Why is WFI stored above 70 °C?
Hot, continuously circulating water stops bacteria from growing and forming biofilm in the pipes. EU GMP Annex 1 gives constant circulation above 70 °C as an example of how to minimise microbial growth in WFI systems.
