Definition: A diluent is the liquid used to dissolve a powdered drug or thin a liquid one before it is injected. The most common diluents are bacteriostatic water and sterile water for injection, but some products use water with salt or a preservative. Labels name a specific diluent, and its volume sets the final concentration.
Diluents named on approved labels
The diluent is not always plain water. These are taken from current US labels:
Diluent supplied with each product, per its label.
| Product | Diluent | Volume |
|---|
| Novarel (hCG) | Bacteriostatic water, 0.9% benzyl alcohol | 10 mL vial |
| Pregnyl (hCG) | Water with 0.56% sodium chloride and 0.9% benzyl alcohol | 10 mL vial |
| Omnitrope 5.8 mg vial (somatropin) | Bacteriostatic water, 1.5% benzyl alcohol | 1.14 mL vial |
| Egrifta SV 2 mg (tesamorelin) | Sterile water for injection, no preservative | 0.5 mL used |
Egrifta SV states it directly: use only the diluent provided. The diluent is part of what was tested.
Why it matters when you track doses
Two facts about the diluent change your log. The type decides whether the vial can be used more than once: a preserved diluent allows repeat draws, and plain sterile water is single-dose. The volume decides the concentration, and so the volume of every dose.
Pregnyl 10,000-unit vial
- Add the full 10 mL of the supplied solvent.
- Concentration: 10,000 units ÷ 10 mL = 1,000 units/mL.
- Each 0.1 mL then holds 1,000 × 0.1 = 100 units.
- The label allows 1 to 10 mL. Mixed with 5 mL instead, the same 0.1 mL would hold 200 units.
Common mix-ups
- Assuming any sterile liquid will do. Salt, preservative and pH differ between diluents.
- Forgetting to record the volume. Without it, nobody can turn syringe marks back into a dose.
- Using leftover diluent as a second vial. A single-dose diluent is meant to be thrown away after use.
Read more on Dose Track: Bacteriostatic vs sterile water · Peptide reconstitution calculator · hCG guide
General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.
Is diluent just water?+
Often, but not always. Pregnyl uses water with 0.56% sodium chloride and 0.9% benzyl alcohol, and bacteriostatic water contains benzyl alcohol.
Does the diluent change how much drug is in the vial?+
No. It changes the concentration, so it changes how much volume holds a given amount of drug.
Can I use a different diluent than the label says?+
Labels name the diluent the product was tested with, and some, like Egrifta SV, say to use only the one provided. Ask your pharmacist before changing it.