One of the most common vitamin overdoses in cats is Vitamin D. It can be caused by vitamin supplements or medications containing high amounts of vitamin D, but also by ingesting certain plants, rat poison, certain skin creams and even commercial cat food (see case study).
It's also possible to overdose a cat with
poorly balanced diets containing high concentrations of liver, milk, fatty fish, or eggs. (Source)
Since vitamin D is lipophilic (fat soluble) it can accumulate over a long time and lead to a slow onset poisoning. Since vitamin supplement pills have become more potent recently, the ingestion of 1 or more pills might already be too much (depending on dosage).
It would take less than a spoonful of cholecalciferol based rodenticide to poison a dog (source) and based on body weight the dosage for a cat is even smaller.
Read more about vitamin D poisoning in cats.
Another category of supplement with a high risk of overdosing are prenatal vitamins because they usually contain very much iron and can cause iron toxicity in pets.
Ingestion of 20 to 60 mg/kg of elemental iron may cause toxicosis with mild gastrointestinal (GI) signs. Ingestion of >60 mg/kg of elemental iron is considered potentially serious and may result in GI hemorrhage as well as metabolic acidosis and elevated liver enzyme values. Death may result if an animal ingests 100 to 200 mg/kg of elemental iron and does not receive treatment.
See the source for details about how to calculate the amount of elemental iron in a medication or food supplement.
Read more about iron toxicity in pets.
Extra ingredients in supplement pills can also cause poisonings, like
- 5 hydroxytryptophan (5 HTP)
- Caffeine (guarana, green tea)
- Citrus aurantium (synephrine)
- Ephedra (ma huang)
Read the source and more information here.