Why half a capsule still means full emergency
Gel caps concentrate D3 in the oily center — your cat likely ingested most of the 2500 IU when biting through. At 4 kg, that's 625 IU per kilogram, well above the 100 IU/kg risk threshold for cats. The gel base also speeds absorption compared to powder-filled tablets, shortening the window for intervention.
Unlike human medications where halves are often safe, vitamin D3's toxicity isn't linear. Even partial doses disrupt calcium regulation for days. Your cat's current normal behavior is irrelevant — clinical signs lag behind internal damage, and waiting for vomiting could mean irreversible kidney changes.
The silent countdown already started
Blood calcium peaks between 12–72 hours post-ingestion, but cellular damage begins immediately. Vets will start IV calcium binders and fluids preemptively, not reactively. Your clinic might hospitalize her for 48 hours minimum, with blood tests every 12 hours to track trends.
Cats hide illness instinctively, so 'acting fine' is meaningless here. Subtle clues like excessive face-rubbing or litter box changes emerge first. By day three, severe signs like tremors or collapse indicate life-threatening hypercalcemia — treatment at that stage becomes far riskier and more expensive.
How gel caps differ from other forms
The soft shell means your cat absorbed D3 faster than if she'd eaten a dry tablet. Liquid-filled capsules also leave less residue in the mouth, so inducing vomiting (which vets don't recommend here) would retrieve little. The gel's ingredients might include glycerin or gelatin, which aren't toxic but complicate decontamination.
Human supplements often pair D3 with calcium or K2 — check the label for these additives. Some 'bone health' blends are extra dangerous because the calcium amplifies D3's effects. Even if the capsule was pet-specific, the dose is inappropriate for feline physiology at any concentration.
What to bring to the vet clinic
Take the remaining half-cap in its original bottle — the exact formulation guides treatment. Note the time of ingestion; if uncertain, assume the worst-case scenario. Prepare for extended hospitalization costs, as vitamin D toxicity isn't resolved with a single injection.
Mention any prior kidney issues, however minor. Skip home remedies like milk or oil — they won't lower calcium and may interfere with professional care. If the clinic is closed, go to an ER; this isn't a 'wait until morning' situation. Time matters more than distance — choose the nearest capable facility, even if it's not your regular vet.
For a dose worked out against your pet's weight, use the Vitamin D (Cholecalciferol) Toxicity Calculator.