Bacteriostatic Water (BAC Water) Guide: Everything You Need for Peptide Reconstitution (2026)

## The most important supply in your peptide reconstitution kit is not the peptide — it is the water. Every reconstituted peptide vial depends on bacteriostatic water (BAC water) to remain safe for multi-dose use. Using the wrong diluent — sterile water, saline, or tap water — can compromise your medication within hours. This guide covers everything you need to know about selecting, using, and storing bacteriostatic water for [semaglutide](/guides/how-to-reconstitute-semaglutide-mixing-guide), [tirzepatide](/guides/how-to-reconstitute-tirzepatide-mixing-guide), and [retatrutide](/guides/how-to-reconstitute-retatrutide-mixing-guide) reconstitution. ## What Is Bacteriostatic Water? Bacteriostatic water is USP-grade sterile water that contains **0.9% benzyl alcohol** as a bacteriostatic preservative. This preservative does not kill bacteria — it inhibits their growth, keeping the solution safe for repeated needle punctures over a 28-day period. **Key properties:** - **Sterile** at time of manufacture - **Preserved** with 0.9% benzyl alcohol - **pH-neutral** — does not affect peptide stability - **Multi-dose safe** — designed for repeated vial entry ## BAC Water vs Sterile Water vs Saline: The Critical Differences | Property | Bacteriostatic Water | Sterile Water | Normal Saline | |:---|:---|:---|:---| | **Preservative** | Yes (benzyl alcohol) | No | No | | **Multi-dose safe** | Yes (28 days) | No (single use) | No (single use) | | **Peptide compatible** | Yes | Yes | Not recommended | | **Cost** | ~$3-5 per 30ml vial | ~$2-3 per vial | ~$2-3 per vial | | **Best for** | Compounded peptides | Single-dose injections | IV infusions | **The bottom line:** If you are drawing multiple doses from one vial over days or weeks, **you must use bacteriostatic water**. No exceptions. ## How Much BAC Water for Each Peptide ### Semaglutide | Vial Size | BAC Water | Concentration | 0.25mg = | 0.5mg = | 1mg = | |:---|:---|:---|:---|:---|:---| | 2mg | 0.8ml | 2.5mg/ml | 10 units | 20 units | 40 units | | 3mg | 1.2ml | 2.5mg/ml | 10 units | 20 units | 40 units | | 5mg | 2.0ml | 2.5mg/ml | 10 units | 20 units | 40 units | Full guide: [How to Reconstitute Semaglutide](/guides/how-to-reconstitute-semaglutide-mixing-guide) ### Tirzepatide | Vial Size | BAC Water | Concentration | 2.5mg = | 5mg = | 10mg = | |:---|:---|:---|:---|:---|:---| | 5mg | 0.5ml | 10mg/ml | 25 units | 50 units | — | | 10mg | 1.0ml | 10mg/ml | 25 units | 50 units | 100 units | | 30mg | 1.0ml | 30mg/ml | 8 units | 17 units | 33 units | | 30mg | 3.0ml | 10mg/ml | 25 units | 50 units | 100 units | Full guide: [How to Reconstitute Tirzepatide](/guides/how-to-reconstitute-tirzepatide-mixing-guide) ### Retatrutide | Vial Size | BAC Water | Concentration | 1mg = | 4mg = | 8mg = | |:---|:---|:---|:---|:---|:---| | 10mg | 1.0ml | 10mg/ml | 10 units | 40 units | 80 units | | 30mg | 3.0ml | 10mg/ml | 10 units | 40 units | 80 units | | 60mg | 6.0ml | 10mg/ml | 10 units | 40 units | 80 units | Full guide: [How to Reconstitute Retatrutide](/guides/how-to-reconstitute-retatrutide-mixing-guide) ## How to Use Bacteriostatic Water Correctly ### Step 1: Check the Vial - Verify the expiration date - Confirm the seal is intact - Solution should be clear and colorless ### Step 2: Clean the Stopper Swab the rubber stopper with an alcohol pad. Let it air dry for 30 seconds before puncturing. ### Step 3: Draw the Correct Volume Use a fresh syringe each time. Draw the exact amount specified for your peptide and vial size (see charts above). ### Step 4: Add to Peptide Vial Insert the needle through the peptide vial stopper. **Aim at the glass wall**, not the powder. Release the water slowly, letting it trickle down the side. ### Step 5: Mix Gently Roll the vial between your palms or swirl gently. **Never shake.** The peptide should dissolve within 2-5 minutes into a clear solution. ## Storage Rules for Bacteriostatic Water **Unopened BAC water:** - Store at room temperature (59-86°F / 15-30°C) - Protect from direct sunlight - Shelf life: check expiration date on vial (typically 2+ years) **Opened BAC water:** - Store at room temperature - Discard after 28 days - Do NOT refrigerate (can cause benzyl alcohol to precipitate) - Keep away from direct light **Reconstituted peptide (BAC water + peptide):** - Refrigerate immediately (36-46°F / 2-8°C) - Use within 28-30 days - NEVER freeze ## Common Mistakes to Avoid 1. **Using sterile water for multi-dose vials** — Without preservative, bacteria can grow within 24 hours. 2. **Reusing syringes** — Each draw should use a fresh, sterile syringe to prevent contamination. 3. **Refrigerating unopened BAC water** — Room temperature is correct for BAC water. Only refrigerate after adding it to your peptide. 4. **Using expired BAC water** — The benzyl alcohol preservative degrades over time. Always check dates. 5. **Drawing from the same BAC water vial for months** — Discard after 28 days, even if there is water remaining. ## What to Look for When Buying BAC Water - **USP grade** — Meets United States Pharmacopeia standards - **0.9% benzyl alcohol** — Standard preservative concentration - **Sealed, sterile vials** — 10ml or 30ml sizes are most practical - **Clear labeling** — Manufacturer, lot number, expiration date - **Reputable source** — Compounding pharmacy or licensed medical supply company **Avoid:** bulk containers, non-pharmaceutical sources, products without lot numbers, or any vial that appears already opened. --- *This guide is for educational purposes only and does not constitute medical advice. Always follow your healthcare provider's specific reconstitution instructions. For dosing guidance, see our [semaglutide dosing guide](/guides/semaglutide-dosing-schedule-week-by-week-guide), [tirzepatide dosing guide](/guides/tirzepatide-dosing-schedule-complete-guide), or [retatrutide dosing guide](/guides/retatrutide-dosing-protocol-complete-titration-guide). Last updated: March 2026.*