What Is Bacteriostatic Water? - Definition & Uses

Instructor: Taormina Lepore

Taormina has taught advanced high school biology, is a science museum educator, and has a Master's degree in museum paleontology.

Water can contain bacteria, but rather than killing off bacteria, bacteriostatic water prevents it from reproducing. In this lesson, we'll discuss the definition, uses, and cautions of bacteriostatic water.

What Is Bacteriostatic Water?

Think of a patient in a hospital bed, clear tubes hooked up to their arm. You've probably seen the familiar clear liquid that runs through these tubes. Hospitals inject patients with all sorts of life-sustaining fluids, from saline for rehydration, to glucose to manage blood sugar levels. These clear fluids can be found hanging from bags by a patient's bedside.

All kinds of medicines can also be injected from a side tube to treat a patient. And each of these medicinal injections rely on an important type of sterile water: bacteriostatic water.

The familiar drip bags and clear tubing, into which medicine can be injected
intravenous drip bag

Bacteriostatic water is water that has been made to inhibit the growth of most types of bacteria. It is comprised of sterile and filtered water, with all bacteria removed, which is then mixed with 0.9% benzyl alcohol, which prevents any contaminating bacteria from growing in the water. In this way, the water has become 'static', or relatively unchanging in its bacterial content.

Bacteriostatic water is used to dilute or dissolve medications for patient injection. It is different from sterile water, which is filtered and purified but has no additives, and is usually only available for single-use.

Bacteriostatic water is not used for drinking or use in drip bags

Why Use Bacteriostatic Water?

Bacteriostatic water can be injected in three main ways:

  1. Intravenous injection is performed by accessing a vein with a needle.
  2. Intramuscular injection accesses a muscle with a needle.
  3. Subcutaneous injection means under the skin, with the root sub- meaning beneath, and cutaneous pertaining to the skin.

Each method is used based on the type of medication and the best path for it to be effective, as determined by a medical professional.

You might ask yourself, why doesn't bacteriostatic water just use an agent that kills bacteria outright? It's true, bacteriostatic water doesn't contain an anti-bacterial agent like some other fluids for injection, so it doesn't kill bacteria off - it just prevents bacteria from growing in the water. But anti-bacterial agents can have adverse reactions to medications, while benzyl alcohol doesn't. Therefore, bacteriostatic water can be used for a broader array of drug dilutions or injections without negatively interacting with bacteria-destroying additives.

Sterile and bacteriostatic water contain no bacteria, but once they are used for the first time (usually by the insertion of a syringe) the container can become contaminated. This is why sterile water can only be used once. The advantage of bacteriostatic water is that bacteria that might contaminate it simply can't grow and reproduce, or act as an infectious agent, and so can be used repeatedly.

Most manufacturers of bacteriostatic water recommend that any unused water be discarded after 28 days, just in case the benzyl alcohol has lost any of its efficiency in preventing bacterial growth over time.

Sometimes bacteriostatic water is made to inhibit bacterial growth using a salt solution instead of benzyl alcohol. The reasons depend on why the water is being used.

When Not to Use Bacteriostatic Water

Sometimes using bacteriostatic water can be harmful.

If you inject bacteriostatic water that is not mixed with a compound, it may cause some blood cells to burst, a condition known as hemolysis ('hemo-' means blood, and 'lysis' is bursting or splitting). This is because unmixed bacteriostatic water is not isotonic, meaning it won't have particles that are balanced with those in the human bloodstream. Bacteriostatic water is therefore not recommended for those clear, direct intravenous drip bags.

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