ProZinc is good for 60 days after opening if you have the 10 mL vial, or 80 days if you have the larger 20 mL vial. These timeframes come directly from the manufacturer, Boehringer Ingelheim, and apply as long as the vial is stored properly in the refrigerator. Even if insulin remains in the vial after that window, it should be discarded.
Why Vial Size Affects the Timeline
The 10 mL vial has a 60-day use window from the first puncture, while the 20 mL vial gets 80 days. This difference exists because larger vials are designed with slightly different formulation or preservative considerations to account for more needle punctures and a longer draw-down period. Regardless of how much insulin is left inside, the clock starts the moment you first insert a syringe into the rubber stopper.
A practical tip: write the date of first use on the vial with a permanent marker. It’s easy to lose track of when you opened it, especially when you’re managing daily injections for your cat or dog over weeks at a time.
How to Store an Open Vial
ProZinc should be kept upright in the refrigerator at 36 to 46°F (2 to 8°C) at all times, both before and after opening. Keep it away from the freezer compartment. If the insulin freezes at any point, it’s ruined and needs to be thrown away. The vial should also be protected from light, so storing it in its original box inside the fridge is ideal.
Avoid placing the vial in the fridge door, where temperatures fluctuate more with repeated opening and closing. A spot toward the middle or back of a shelf, away from the cooling element, provides the most stable temperature.
Signs the Insulin Has Gone Bad
ProZinc is a suspension, so it naturally looks somewhat cloudy or milky when mixed properly. That’s normal. What you’re watching for are changes from its usual appearance: unusual discoloration, visible clumps or particles that won’t dissolve with gentle rolling, or a frosted look on the inside of the glass. If the insulin looks different from what you’re used to seeing, discard it even if it’s within the 60- or 80-day window.
Insulin that has been exposed to temperature extremes can lose potency without looking visibly different. If your cat’s blood sugar suddenly becomes harder to control and nothing else has changed, a degraded vial is worth considering as the cause.
Mixing Before Each Dose
Because ProZinc is a suspension, the active ingredient settles to the bottom of the vial between uses. Before drawing a dose, gently roll the vial between your palms to resuspend the contents evenly. Do not shake it vigorously. Shaking creates air bubbles that make it difficult to draw an accurate dose and can damage the insulin molecules, reducing potency over time.
Traveling With ProZinc
If you need to transport ProZinc for travel or a vet visit, keep it cool without freezing it. A small insulated bag with a cool pack works well, but place a cloth or towel between the cool pack and the vial so the insulin doesn’t freeze on contact. Avoid leaving it in a hot car, near a window, or in direct sunlight, as heat degrades insulin quickly.
For air travel, pack insulin in your carry-on luggage. Checked baggage compartments can reach freezing temperatures, and you also risk losing access to it if bags are delayed. The 60- or 80-day use window still applies during travel, so factor trip length into your planning.
What Happens if You Use Expired Insulin
Insulin that has passed its use-by window doesn’t become toxic, but it gradually loses potency. The protamine zinc formulation breaks down over time, meaning each dose delivers less blood sugar control than expected. For a diabetic pet, this can lead to persistent hyperglycemia: increased thirst, frequent urination, weight loss, and lethargy. These symptoms can creep in gradually, making it easy to attribute them to disease progression rather than a vial that’s past its prime.
Replacing the vial on schedule is one of the simplest ways to keep your pet’s diabetes well managed. Given that a single vial often lasts well within the 60- or 80-day window for most dosing schedules, timing rarely becomes an issue, but it’s worth tracking nonetheless.