How to Switch from a GLP-1 Pen to a Compounded Vial

When you switch from a GLP-1 pen to a compounded vial, the number that carries over is your dose in milligrams, not a number of units. Whether you keep that dose or step down is a decision for your clinical team, based on how well you tolerate your current dose, because no study has settled the best way to switch. The most common problem is the math. The same number of units on a syringe can be double the dose in a stronger vial.

Should my dose change when I switch from a pen to a vial?

Your pen sets your dose in milligrams. With a vial, you work out the volume yourself in units on an insulin syringe, as JumpstartMD's guide to measuring compounded semaglutide explains. So your milligram dose is the starting point for your vial. Units measure volume, and how many units you need depends on how strong your vial is.

Whether you stay at that milligram dose is less settled. According to a report on switching between incretin medicines in clinical practice, there are no outcomes data on moving between these medicines. A 2020 consensus suggested starting at the lowest dose, or at a slightly lower or equal dose if side effects aren't a concern. Manufacturers recommend the lowest dose. The published dose tables vary drastically. Because the advice disagrees, your clinical team makes the call based on how you're handling your current dose.

How you're handling it matters because stomach side effects rose with dose in the Zepbound weight-loss trials. Severe stomach side effects were reported in 1.7% of people on 5 mg, 2.5% on 10 mg and 3.1% on 15 mg, compared with 1% on placebo, according to the Zepbound label. If your current dose still upsets your stomach, tell your clinical team before you switch.

The same label says the dose may go up in 2.5 mg steps after at least 4 weeks on the current dose. If you can't tolerate a maintenance dose, it says to consider a lower one. Moving to a vial is no reason to raise your dose faster.

One woman described making this switch with her doctor's support. In a r/tirzepatidecompound thread, she wrote: "F65 here who was prescribed Ozempic, quickly transitioned to Compounded Tirzepatide with the support of my PCP."

How do I convert my pen dose in mg to units on a syringe?

1. Find the concentration printed on your vial in mg/mL. FDA notes that strength varies between compounders, and one compounder may sell several strengths. Never assume your new vial matches your last one.
2. Use a U-100 insulin syringe and do the math. On a U-100 syringe, 100 units equals 1 mL. The formula is: units = (your dose in mg ÷ vial concentration in mg/mL) × 100.
3. Check your answer against a worked example. In a 5 mg/mL vial, 0.5 mg is 10 units, 1.0 mg is 20 units and 1.7 mg is 34 units. In a 2.5 mg/mL vial, 20 units is 0.5 mg. That means the same 20 units from a 5 mg/mL vial is twice the dose.
4. Ask your clinical team to write your dose in milligrams and in units for your vial, and to confirm the syringe size. FDA asks prescribers and compounders to provide the right syringe size and show patients how to measure the dose.
5. Redo the math every time you get a new vial, even if your dose hasn't changed.

What mistakes cause overdoses after switching to vials?

FDA has received reports of people overdosing on compounded semaglutide. Most described patients drawing more than their prescribed dose from a multiple-dose vial and injecting 5 to 20 times the intended dose. Some needed hospital care.

In several reports, patients told to draw 5 units drew 50. Prescribers have made mistakes too. One meant to prescribe 0.25 mg (5 units) but wrote 25 units, and the patient had severe vomiting.

The reported effects of these overdoses include nausea, vomiting, belly pain, fainting, headache, migraine, dehydration, acute pancreatitis (a sudden inflammation of the pancreas) and gallstones. Semaglutide stays in the body for a long time. Its half-life is about one week, which means it takes about a week for half of a dose to clear. Someone who has taken too much may need to be watched for a while.

In FDA's adverse event database from 2018 to 2024, reports involving compounded GLP-1s were more likely than reports for brand products to mention preparation errors, contamination and hospitalization. These are reports, not proof that the medicine caused the problem. They are a good reason to double-check your vial and your math.

What should I check on a compounded vial before my first dose?

• Know what compounded means. FDA says compounded drugs should only be used when a patient's medical needs can't be met by an FDA-approved drug, or when the approved drug isn't commercially available. FDA does not review compounded drugs for safety, effectiveness or quality before they are sold.
• For semaglutide, ask whether the vial contains semaglutide base. FDA says salt forms such as semaglutide sodium and semaglutide acetate are different active ingredients from the approved drug. It knows of no lawful basis for compounding with them.
• Check for added ingredients such as B12, B6, L-carnitine or NAD. FDA says the safety and effectiveness of combining semaglutide with these has not been established.
• Check how it arrived and how you store it. Don't use a vial that arrives warm. Use a new sterile needle and syringe for every dose, and throw the vial away within 28 days of first use, even if there is medicine left.
• Watch for red flags. These include a seller claiming the compounded drug is the same as the brand, deep discounts, no screening or prescription by a licensed doctor, and a label with misspellings or a pharmacy name you can't verify.
• Tell your clinical team if your new vial feels different from your pen.

When do I take my first vial dose, and what should I watch for?

If you take tirzepatide, the Zepbound label lets you change your weekly injection day as long as your doses stay at least 3 days (72 hours) apart. That helps if your vial arrives off-schedule.

If you take birth control pills, the label advises switching to a non-oral method or adding a barrier method, such as condoms, for 4 weeks after you start tirzepatide and for 4 weeks after each dose increase. That applies if tirzepatide is new to you or if your dose goes up with the switch.

Tell your clinical team right away if your stomach symptoms are severe or don't let up. The label also notes reports of kidney injury, mostly in people who became dehydrated from nausea, vomiting or diarrhea.

Check in with your clinical team after your first vial dose and again after your first full vial. That way, any change to your dose is one you planned.

Take the quiz to tell our clinical team which pen and dose you're on now. Our clinical team can prescribe semaglutide or tirzepatide if it fits your goal. They can also help you carry your dose into a plan you can measure with confidence.