By Priya Mehta, PharmD, Clinical Pharmacist. Medically reviewed by Dr. Anika Rao, MD, Board-Certified Internal Medicine.
Last month, a woman named Rachel in Scottsdale called her pharmacy in a mild panic. She'd been injecting compounded tirzepatide at 2.5 mg weekly for three weeks without issue. Then the refill arrived with a different concentration on the label: 10 mg/mL instead of 5 mg/mL. She drew the same 50 units she'd been drawing all along, which meant she accidentally doubled her dose to 5 mg. "I spent that night on the bathroom floor," she told the pharmacist. "Nobody warned me the concentration could just... change."
Rachel's mistake is the single most common dosing error in self-administered compounded GLP-1 protocols. This guide exists to prevent it.
This article is part of the FormBlends ultimate guide to compounded tirzepatide and the Tirzepatide Dosing & Protocols hub.
The short version
- The formula: dose in mg ÷ concentration in mg/mL = volume in mL. Multiply that volume by 100 to get the number on your U-100 syringe.
- The unit count changes every time the pharmacy ships a different concentration, even if your prescribed milligram dose stays the same. This is where most errors happen.
- Standard tirzepatide escalation per SURMOUNT-1: 2.5 mg weekly for four weeks, then 5 mg, with step-ups every four weeks as tolerated.
- Compounded tirzepatide is not FDA-approved. The FDA does not pre-review compounded medications for safety, effectiveness, or quality.
Before you uncap anything
Gather your supplies: the current pharmacy label (read it, don't just glance at it), a U-100 insulin syringe in the right barrel size (typically 0.3 mL or 0.5 mL for compounded GLP-1 dosing), an alcohol prep pad, a sharps container, your prescribed dose written down somewhere you can see it, and your prescriber's contact info.
Two numbers on that label matter more than everything else combined: the concentration in mg/mL and the total fill volume. The concentration drives the math. The fill volume tells you how many doses live in the vial.
Check the medication name, the patient name, the expiration date, and the storage instructions before every injection. If anything looks off, don't inject. Call the pharmacy. This isn't overcaution; it's the baseline.
The math (and why you should write it down)
Here's the thing about this calculation: it's simple enough that people try to do it in their heads, and simple enough that they sometimes get it wrong.
Check your GLP-1 eligibility
Use our free BMI Calculator to see if you may qualify for provider-reviewed GLP-1 therapy.
Try the BMI Calculator →Take your prescribed dose in milligrams. Divide by the concentration on the label in mg/mL. The result is the volume you need to draw, in mL. Multiply that number by 100 to translate it into units on a U-100 syringe.
Example at 5 mg/mL concentration: 2.5 mg ÷ 5 mg/mL = 0.5 mL = 50 units
Example at 10 mg/mL concentration: 2.5 mg ÷ 10 mg/mL = 0.25 mL = 25 units
Same milligram dose. Half the units. That's the difference that sent Rachel to the bathroom floor.
Write your result down on paper or in your phone. Do not rely on memory, especially if multiple family members inject different doses from different vials. If the math produces a fractional unit (say, 12.5 units), confirm the rounding direction with your prescriber before injecting. Don't improvise.
A decent habit: stick a small label on the vial cap itself at first fill with the prescribed dose in mg and the corresponding unit count. The number is visible every time the cap comes off.
The thirty-second check that catches half of all errors
Before you draw anything into the syringe, read the dose back to yourself out loud. Better yet, read it to someone else in the room, or call the pharmacy. This sounds tedious. It takes thirty seconds.
In self-administered medication protocols, this single verification step catches roughly half the errors that would otherwise make it into the syringe. It is, per unit of effort, the most valuable thing you can do in the entire process.
The injection itself
Clean the injection site with the alcohol prep pad. Wait for it to dry completely (wet alcohol stings, and the antiseptic effect requires air drying). Pinch a fold of skin if needed depending on subcutaneous fat depth at the site. Insert the needle at 90 degrees, push the plunger smoothly without jerking, count to five with the needle still in place, then withdraw and drop it straight into the sharps container.
Rotate sites across weeks: abdomen, front of the thigh, back of the upper arm. Stay at least two inches from the navel. Do not inject through clothing. Repeating the same spot week after week creates nodules and lipohypertrophy, which changes absorption and makes dosing unpredictable. Think of it like rotating tires: boring maintenance that prevents a real problem.
Related reading
Keep a log (the boring truth about long-term success)
Record five things after each injection: the date, the dose in milligrams, the units drawn, the injection site, and any symptoms.
A single injection tells you very little. A log spanning eight or twelve weeks tells you a lot. It reveals trends in tolerability, patterns of side effects at specific doses, and gaps in adherence you might not notice otherwise. Review the log monthly with your prescriber. Across the GLP-1 class, the strongest predictor of long-term outcomes is sustained months on therapy at or near the maintenance dose. Everything else matters less than consistency.
Related reading from adjacent topics
The three mistakes that keep recurring
Mistake 1: Recycling last month's unit number after a concentration change. This is Rachel's error. New vial, new label, new math. Every single time.
Mistake 2: Same-site repetition. People find a comfortable spot and stick with it (literally). Rotate. Mark your log if it helps.
Mistake 3: Temperature drift. Compounded peptides are temperature-sensitive. Leaving a vial on a kitchen counter for an afternoon, in a hot car, or next to a freezer vent can compromise the formulation. When in doubt, call the pharmacy before injecting from that vial.
When to stop and call someone
Seek emergency care for: severe abdominal pain (especially radiating to the back, which can signal pancreatitis), persistent vomiting preventing fluid intake, jaundice or right-upper-quadrant pain suggesting gallbladder disease, signs of allergic reaction (rash, facial or throat swelling, difficulty breathing), severe dehydration, or thoughts of self-harm.
Call your prescriber within 24 to 48 hours for: nausea persisting beyond two weeks at a stable dose, new vision changes, ongoing constipation despite adequate hydration and fiber, or any new symptom you can't explain.
For non-urgent questions about timing, routine side effects, or whether to adjust: schedule a follow-up instead of self-adjusting. Dose escalation is protocol-driven, and freelancing the schedule is where people run into trouble.
A note on trial data and expectations
SURMOUNT-1 reported substantial variation in response within the same dose arm. That's the normal pattern across GLP-1 trials. Trial averages compress enormous individual variance into a single headline number. Reading the published distribution behind the average is more useful than fixating on the average itself.
Real-world cohorts add even more variance, primarily from adherence and lifestyle differences. The trial number is a useful anchor, not a guaranteed destination.
Frequently asked questions
Is this question something I should discuss with a clinician?
Yes. Any question that affects how a prescription medication is dosed, stored, or administered is worth raising with your prescriber. This article is general education, not a substitute for individualized clinical guidance.
What if I drew the wrong number of units last week?
Stop. Do not inject again. Contact your prescriber. If the medication was already injected at the wrong dose, your prescriber needs to know so they can advise on timing and next steps. Do not try to "make up" a dose without explicit instructions.
Does the unit count change if the pharmacy ships a different concentration?
Yes. The unit count is derived from the concentration. A new concentration means a new unit count, even if the milligram dose on your prescription hasn't changed. Always re-read the label.
Can I use a different syringe than the one the pharmacy provided?
Use only the syringe size and type specified by your pharmacy. A U-100 insulin syringe is standard for most compounded GLP-1 concentrations. Substituting a different syringe changes the calibration and is a known source of dosing errors.
Is compounded tirzepatide FDA-approved?
No. Compounded tirzepatide is not an FDA-approved drug. The FDA does not review compounded medications for safety, effectiveness, or quality prior to dispensing. Compounded medications are dispensed under personalized prescriptions through state-licensed pharmacies when a prescriber determines a personalized formulation is clinically appropriate.
Continue the series
Important Safety Information
This article is for educational purposes only and is not medical advice. Compounded tirzepatide and compounded semaglutide are not FDA-approved drugs. The FDA does not review compounded medications for safety, effectiveness, or quality before they are sold. Compounded medications should only be used when a licensed prescriber determines a personalized formulation is clinically appropriate. Do not start, stop, or modify any prescription medication without speaking with a licensed healthcare provider. If you experience symptoms of a serious reaction, including severe abdominal pain, signs of pancreatitis, vision changes, persistent vomiting, signs of an allergic reaction, or thoughts of self-harm, seek emergency care immediately.
FormBlends sells only compounded semaglutide and compounded tirzepatide through licensed U.S. pharmacies after a telehealth evaluation by an independent prescriber. Eligibility, pricing, and formulation are determined on a case-by-case basis.
About this article
Written by Priya Mehta, PharmD (Clinical Pharmacist). Medically reviewed by Dr. Anika Rao, MD (Board-Certified Internal Medicine). FormBlends content is reviewed by licensed U.S. clinicians prior to publication. The clinical decisions described above are general education only and should not replace individualized advice from your own healthcare provider.