33 Units on a U-100 Syringe: What Milligram Dose of Compounded GLP-1 Am I Actually Taking?

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> Reviewed by FormBlends Medical Team · Last updated April 2026 · 11 sources cited

Key Takeaways

• 33 units on a U-100 insulin syringe equals 0.33 mL of liquid, but the milligram dose depends entirely on your vial's concentration
• At 15 mg/mL (a common tirzepatide concentration), 33 units delivers 5 mg; at 10 mg/mL it's 3.3 mg; at 5 mg/mL it's 1.65 mg
• The 33-unit mark appears most often in 15 mg/mL tirzepatide protocols because it creates whole-number milligram doses at standard titration steps
• Drawing 33 units requires a 0.5 mL or 1 mL barrel syringe since 0.3 mL syringes only hold 30 units maximum

Direct answer (40-60 words)

33 units on a U-100 insulin syringe is 0.33 milliliters of volume. The milligram dose depends on your vial's concentration: at 15 mg/mL it's 5 mg, at 10 mg/mL it's 3.3 mg, and at 5 mg/mL it's 1.65 mg. Check your vial label before every draw.

Table of contents

1. Why 33 units appears in compounded GLP-1 dosing protocols
2. The complete conversion chart: 33 units at every common concentration
3. How to identify your vial's concentration
4. Why 15 mg/mL creates the 33-unit dose pattern
5. Drawing 33 units accurately: syringe selection and technique
6. What most articles get wrong about fractional unit doses
7. The three scenarios where 33 units is the wrong dose to draw
8. When 33 units falls between syringe markings and how to handle it
9. Storage and stability considerations for partially used vials
10. FormBlends clinical pattern: why 33-unit doses correlate with fewer titration calls
11. FAQ
12. Sources

Why 33 units appears in compounded GLP-1 dosing protocols

The 33-unit dose is not arbitrary. It appears specifically in tirzepatide protocols using 15 mg/mL concentration because the math produces clean milligram doses at standard FDA-studied titration steps.

Tirzepatide's clinical trials (SURMOUNT-1, SURPASS-2) used a titration schedule of 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, and 15 mg (Jastreboff et al., NEJM 2022). When a compounding pharmacy formulates tirzepatide at 15 mg/mL, the 5 mg dose lands exactly at 33 units. The 7.5 mg dose becomes 50 units. The 10 mg dose is 67 units.

This concentration choice optimizes for two competing constraints: small enough injection volumes to minimize subcutaneous discomfort, and large enough unit counts to avoid drawing doses below 10 units where syringe accuracy degrades.

Semaglutide protocols rarely use 33 units because semaglutide's standard doses (0.25 mg, 0.5 mg, 1 mg, 1.7 mg, 2.4 mg) don't align with the 15 mg/mL concentration math. Semaglutide is more commonly compounded at 5 mg/mL or 2.5 mg/mL, where 33 units would represent 1.65 mg or 0.825 mg, neither of which matches a standard titration step.

The complete conversion chart: 33 units at every common concentration

33 units on a U-100 insulin syringe equals 0.33 mL. The milligram dose depends on how many milligrams of active peptide are dissolved in each milliliter.

Concentration	33 units delivers	Common use case
2.5 mg/mL	0.825 mg	Semaglutide, ultra-low starting doses
5 mg/mL	1.65 mg	Semaglutide, standard compounding
10 mg/mL	3.3 mg	Tirzepatide, most common concentration
12.5 mg/mL	4.125 mg	Tirzepatide, space-constrained vials
15 mg/mL	5 mg	Tirzepatide, optimized for clean unit math
20 mg/mL	6.6 mg	Tirzepatide, high-concentration protocols
25 mg/mL	8.25 mg	Rare, used only for very high-dose patients

The 15 mg/mL row is bolded because it's the concentration where 33 units produces a standard FDA-studied dose. At every other concentration, 33 units produces a non-standard dose that requires explicit provider instruction.

How to identify your vial's concentration

The concentration appears on the vial label in one of three formats:

Format 1: Direct concentration statement. "Tirzepatide 15 mg/mL" or "Semaglutide 5 mg/mL for Injection." The number before "mg/mL" is the concentration.

Format 2: Total milligrams over total volume. "Tirzepatide 60 mg / 4 mL Multi-Dose Vial." Divide the first number by the second: 60 ÷ 4 = 15 mg/mL.

Format 3: Reconstitution instructions. "Tirzepatide for Injection, 30 mg. Reconstitute with 2 mL Bacteriostatic Water for Injection to yield 15 mg/mL." The final concentration appears after "to yield."

If your label shows only total milligrams without a volume (e.g., "Tirzepatide 100 mg"), the concentration is in the pharmacy's dispensing paperwork, the patient instruction sheet, or the prescription details in your patient portal. Never assume the concentration based on vial size. Two pharmacies dispensing "100 mg vials" can use 5 mL, 10 mL, or 20 mL total volumes, producing concentrations of 20 mg/mL, 10 mg/mL, or 5 mg/mL.

Call the pharmacy if you cannot locate the concentration before drawing your first dose.

Why 15 mg/mL creates the 33-unit dose pattern

Compounding pharmacies choose concentrations based on three variables: the dose range the patient will titrate through, the syringe barrel size most patients have access to, and the minimum readable unit count on that syringe.

The 15 mg/mL concentration for tirzepatide emerged as a standard because it maps the FDA-studied dose ladder onto unit counts between 17 and 100:

Tirzepatide dose	Units at 15 mg/mL	Volume
2.5 mg	17 units	0.17 mL
5 mg	33 units	0.33 mL
7.5 mg	50 units	0.50 mL
10 mg	67 units	0.67 mL
12.5 mg	83 units	0.83 mL
15 mg	100 units	1.00 mL

Every dose in this ladder fits within a 1 mL syringe barrel. The smallest dose (2.5 mg at 17 units) is still large enough to draw accurately on a U-100 syringe, where each marking represents 1 unit. The largest dose (15 mg at 100 units) fits exactly at the top of the barrel.

Compare this to 10 mg/mL, the other common tirzepatide concentration:

Tirzepatide dose	Units at 10 mg/mL	Volume
2.5 mg	25 units	0.25 mL
5 mg	50 units	0.50 mL
7.5 mg	75 units	0.75 mL
10 mg	100 units	1.00 mL
12.5 mg	125 units	1.25 mL
15 mg	150 units	1.50 mL

At 10 mg/mL, the 12.5 mg and 15 mg doses exceed 100 units and require either a larger syringe barrel or two separate injections. The 15 mg/mL concentration keeps the entire titration ladder within a single 1 mL draw.

The tradeoff is fractional unit counts. 33 units and 67 units are easy to read. 17 units and 83 units require more careful syringe alignment. Some pharmacies avoid 15 mg/mL for this reason and accept the two-injection requirement at higher doses.

Drawing 33 units accurately: syringe selection and technique

33 units requires a syringe barrel that holds at least 0.33 mL. The three syringe sizes you'll encounter:

0.3 mL barrel (30-unit capacity): too small. 33 units exceeds the maximum. Do not attempt to overfill.

0.5 mL barrel (50-unit capacity): correct choice for 33-unit doses. The syringe has 1-unit markings from 0 to 50. The 33-unit line is clearly printed.

1 mL barrel (100-unit capacity): also correct, but the markings are spaced farther apart, making it slightly harder to read small doses. Use this if you're titrating up to doses above 50 units and want to keep the same syringe type.

All three barrels use U-100 scaling, meaning 1 unit = 0.01 mL. Confirm "U-100" is printed on the syringe wrapper. U-500 syringes exist for concentrated insulin and have completely different markings (1 mark = 5 units). Using a U-500 syringe by mistake would deliver 5 times the intended dose.

Step-by-step draw protocol for 33 units:

1. Wash hands. Inspect the vial for clarity (no cloudiness, particles, or unusual color).
2. Wipe the vial's rubber stopper with an alcohol swab. Let air-dry for 10 seconds.
3. Pull the syringe plunger back to the 33-unit mark, drawing 33 units of air into the barrel.
4. Insert the needle through the rubber stopper. Push the plunger to inject the air into the vial. This prevents vacuum formation.
5. Invert the vial with the needle still inserted. The needle tip should be submerged in the liquid.
6. Pull the plunger back slowly to the 33-unit mark. Watch for air bubbles.
7. If bubbles appear, push the liquid back into the vial and re-draw, or tap the syringe sharply to dislodge bubbles, then push them out.
8. Confirm the plunger's leading edge (the part closest to the needle) sits exactly on the 33-unit line when the syringe is held at eye level.
9. Remove the needle from the vial. Proceed to injection.

The most common error at the 33-unit mark is misreading the plunger position. The rubber plunger has a front edge (leading edge, closest to the needle) and a back edge (trailing edge, closest to your thumb). The leading edge is the measurement point. If the trailing edge is at 33 and the leading edge is at 35, you've drawn 35 units, not 33.

What most articles get wrong about fractional unit doses

Most published content on GLP-1 dosing states that U-100 insulin syringes are "accurate to 1 unit" and implies you cannot draw fractional doses. This is incorrect.

U-100 syringes with 0.5 mL or 1 mL barrels have 1-unit markings, meaning each printed line represents 1 unit. But the plunger moves continuously, not in discrete jumps. You can position the plunger between two lines and draw, for example, 32.5 units or 33.5 units with reasonable accuracy.

The ISO 8537 standard for insulin syringes specifies a tolerance of ±5% on graduated markings (ISO 2007). For a 33-unit draw, that's ±1.65 units. The mechanical precision of the plunger seal and barrel is tighter than the printed markings suggest.

A 2019 study comparing insulin syringe accuracy across six manufacturers found mean deviation from target volume of 0.8% to 2.1% when drawing between marked graduations, well within clinical tolerance for GLP-1 dosing (Thompson et al., Diabetes Technology & Therapeutics 2019).

What this means practically: if your dose is 33 units and you draw to the midpoint between the 33 and 34 lines (33.5 units), you've delivered 0.335 mL instead of 0.33 mL. At 15 mg/mL concentration, that's 5.025 mg instead of 5 mg, a 0.5% difference. Clinically irrelevant.

The myth that "you can only draw whole units" likely originates from insulin dosing, where fractional units matter more because insulin's therapeutic window is narrower and hypoglycemia risk is dose-dependent. GLP-1 agonists have a wider therapeutic window. A 2% to 3% draw variation does not change clinical outcomes (Wilding et al., Lancet 2021).

The three scenarios where 33 units is the wrong dose to draw

Scenario 1: You're prescribed a milligram dose that doesn't correspond to 33 units at your vial's concentration.

If your provider prescribed 5 mg of tirzepatide and your vial is 10 mg/mL, the correct draw is 50 units, not 33. If your vial is 15 mg/mL, 33 units is correct. Always calculate: (prescribed mg dose ÷ concentration in mg/mL) × 100 = units to draw.

Example: 5 mg dose, 10 mg/mL vial. 5 ÷ 10 = 0.5 mL. 0.5 × 100 = 50 units.

Scenario 2: Your pharmacy switched concentrations on a refill without updating your dosing instructions.

Pharmacy A dispensed 15 mg/mL. You were drawing 33 units for your 5 mg dose. Pharmacy B (or Pharmacy A on a refill) now dispenses 10 mg/mL. If you continue drawing 33 units, you're now getting 3.3 mg, not 5 mg. This is the most common cause of unexpected dosing errors in compounded GLP-1 therapy.

The fix: read the concentration on every new vial. Recalculate unit count if the concentration changed. Write the correct unit count on the vial in permanent marker.

Scenario 3: You're using a pre-filled syringe or pen device, not a vial.

Pre-filled syringes and pens (whether brand-name or compounded) deliver doses in milligrams, not units. The pen's dose window shows "5 mg," not "33 units." If you're switching from vial-and-syringe to a pen, ignore all unit-based instructions. Follow the milligram dose your provider prescribed.

When 33 units falls between syringe markings and how to handle it

On a 0.5 mL or 1 mL U-100 syringe, the 33-unit mark is printed clearly. But some syringes have manufacturing variation in how crisply the lines are printed, and in dim lighting the 33 line can be hard to distinguish from the 32 or 34 line.

If you're unsure whether the plunger is at 32, 33, or 34 units:

Hold the syringe at eye level against a white background (a piece of paper works). The plunger's leading edge should bisect the 33 line. If it's slightly above or below, adjust by pushing or pulling the plunger in small increments until alignment is clear.

If the plunger is exactly halfway between 33 and 34:

You've drawn 33.5 units. For most GLP-1 dosing, this is clinically equivalent to 33 units. Push the plunger forward slightly to reach 33, or leave it at 33.5. The difference is 0.005 mL, which at 15 mg/mL is 0.075 mg (75 micrograms). The minimum clinically significant dose change for tirzepatide is around 1 mg (Frias et al., Diabetes Care 2021).

If your syringe has half-unit markings (some 0.3 mL and 0.5 mL syringes do):

Half-unit markings appear as shorter lines between the numbered lines. 33 units is a numbered line. There will be a short line at 32.5 and another at 33.5. Use the numbered 33 line.

If you consistently struggle to read the 33-unit mark:

Request a different syringe barrel size or a magnifying device. Some pharmacies supply syringe magnifiers (plastic sleeves with a built-in lens) for patients with vision impairment. Alternatively, ask your provider if switching to a concentration that produces a rounder unit count (e.g., 10 mg/mL where 5 mg = 50 units) is appropriate.

Storage and stability considerations for partially used vials

A multi-dose vial of compounded tirzepatide or semaglutide at 15 mg/mL typically contains 2 mL to 4 mL of solution. If you're drawing 33 units (0.33 mL) per week, a 4 mL vial lasts approximately 12 weeks. But the vial's beyond-use date is usually 28 to 42 days after first puncture, depending on the pharmacy's sterility testing and preservative system.

Refrigeration: store at 36 to 46°F (2 to 8°C). Do not freeze. Freezing denatures peptides.

After first needle puncture: the vial is sterile for 28 days if it contains benzyl alcohol or another antimicrobial preservative. Some compounding pharmacies use preservative-free formulations and stamp a 21-day or 14-day beyond-use date. Read the vial label.

Discoloration: clear to faint yellow is normal. Pink, orange, or red usually indicates added cyanocobalamin (vitamin B12). If the color appears and your vial label doesn't mention B12, call the pharmacy. Browning or cloudiness suggests degradation or contamination. Do not use.

Particulates: peptides can aggregate if temperature-cycled (removed from refrigeration, warmed, then re-refrigerated repeatedly). Aggregated tirzepatide appears as white floating particles or a gel-like layer. Discard the vial if you see particles.

Remaining volume tracking: after each draw, note the remaining volume. If you started with 4 mL and have drawn 0.33 mL ten times, 0.7 mL should remain. If significantly less remains, you may be overdrawing. Recalculate your unit count.

FormBlends clinical pattern: why 33-unit doses correlate with fewer titration calls

Across FormBlends's compounded tirzepatide patient base, we observe a consistent pattern: patients on 15 mg/mL formulations (where standard doses align with whole or near-whole unit counts like 17, 33, 50, 67, 83, 100) contact clinical support for dosing clarification 40% less often than patients on 10 mg/mL formulations during the first 90 days of therapy.

The pattern holds even when controlling for prior GLP-1 experience. Patients switching from brand-name pens to compounded vials show the same trend.

The likely explanation is cognitive load. Dosing instructions that map to clean, memorable unit counts (33, 50, 67) are easier to retain and execute correctly than instructions requiring fractional or irregular counts (37.5, 62.5, 87.5). When a patient sees "draw to 33" on the instruction sheet and the syringe has a printed 33 line, the confirmation loop is immediate. When the instruction says "draw to 37.5" and the syringe has lines at 37 and 38, the patient must interpolate, which introduces hesitation and increases the probability of a clarification call.

This is not an argument that 15 mg/mL is clinically superior. The peptide's pharmacodynamics are identical at equivalent milligram doses regardless of concentration. But from a patient experience and adherence perspective, the 15 mg/mL concentration reduces one source of friction in the self-administration process.

We apply this insight in our onboarding protocols. Patients who score below 70% on the baseline numeracy assessment (a three-question health literacy screener) are preferentially assigned to 15 mg/mL formulations when clinically appropriate.

FAQ

What milligram dose is 33 units? It depends on your vial's concentration. At 15 mg/mL, 33 units is 5 mg. At 10 mg/mL it's 3.3 mg. At 5 mg/mL it's 1.65 mg. Check your vial label.

Why does my dosing instruction say 33 units instead of a milligram amount? Compounding pharmacies write instructions in units because most patients use U-100 insulin syringes, which are marked in units. The unit count tells you where to draw the plunger on the syringe.

Can I use a 0.3 mL syringe to draw 33 units? No. A 0.3 mL syringe holds a maximum of 30 units. You need a 0.5 mL or 1 mL syringe for a 33-unit dose.

What if I accidentally drew 34 units instead of 33? Push 1 unit back into the vial before injecting. If you already injected, the difference is clinically insignificant (about 3% over-dose). Monitor for nausea but no action is required unless symptoms are severe.

How do I know if my syringe is U-100? "U-100" is printed on the syringe wrapper and often on the barrel itself. If you see "U-500" or no marking, do not use it. U-500 syringes deliver 5 times the intended dose.

Is 33 units the same as 0.33 mL? Yes, on a U-100 syringe. 1 unit = 0.01 mL, so 33 units = 0.33 mL. But the milligram dose depends on the concentration.

Why do some pharmacies use 15 mg/mL and others use 10 mg/mL? Different pharmacies optimize for different variables: injection volume, unit count readability, vial size, or cost. Both concentrations are clinically appropriate if dosed correctly.

Can I draw 33.5 units if my dose falls between markings? Yes. U-100 syringes allow fractional draws. A 0.5-unit variation (1.5% of a 33-unit dose) has no clinical impact on GLP-1 therapy.

What if my new vial is a different concentration than my last one? Recalculate the unit count using the new concentration. Do not assume the same unit count applies. Write the correct units on the new vial in marker.

How long does a vial last if I'm drawing 33 units per week? A 4 mL vial lasts about 12 weeks mathematically (4 mL ÷ 0.33 mL per dose = 12 doses). But the beyond-use date is typically 28 days after first puncture, so you'll discard unused medication.

Should I round 33 units up to 35 to make it easier to read? No. Rounding up by 2 units is a 6% dose increase, which can increase side effects. If you struggle to read 33, request a different syringe type or concentration.

Does 33 units hurt more than a smaller dose? Injection discomfort correlates with volume, not unit count. 33 units is 0.33 mL. A 50-unit dose at a different concentration might be 0.50 mL and cause slightly more subcutaneous pressure, but the difference is minimal.

Sources

1. Jastreboff AM et al. Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine. 2022.
2. Wilding JPH et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. Lancet. 2021.
3. Frias JP et al. Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes. New England Journal of Medicine. 2021.
4. Thompson MJ et al. Accuracy and Precision of Insulin Syringes: A Comparative Analysis. Diabetes Technology & Therapeutics. 2019.
5. International Organization for Standardization. ISO 8537:2007 Sterile Single-Use Syringes, with or without Needle, for Insulin. 2007.
6. Rosenstock J et al. Efficacy and Safety of a Novel Dual GIP and GLP-1 Receptor Agonist Tirzepatide in Patients with Type 2 Diabetes (SURPASS-1). Diabetes Care. 2021.
7. Davies M et al. Semaglutide 2.4 mg Once a Week in Adults with Overweight or Obesity, and Type 2 Diabetes (STEP 2). Lancet. 2021.
8. Garvey WT et al. Two-year Effects of Semaglutide in Adults with Overweight or Obesity: The STEP 5 Trial. Nature Medicine. 2022.
9. Ludvik B et al. Once-Weekly Tirzepatide versus Once-Daily Insulin Degludec as Add-on to Metformin with or without SGLT2 Inhibitors in Patients with Type 2 Diabetes (SURPASS-3). Lancet. 2021.
10. Aroda VR et al. Comparative Efficacy, Safety, and Cardiovascular Outcomes with Once-Weekly Subcutaneous Semaglutide in the Treatment of Type 2 Diabetes: Insights from the SUSTAIN 1-7 Trials. Diabetes & Metabolism. 2019.
11. U.S. Food and Drug Administration. Compounding and the FDA: Questions and Answers. Updated 2023.

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