Does one drop of nicotine kill a horse? There is a popular aphorism in Russia, translated as "one drop of nicotine kills a horse": !enter image description here , which is widely used on the anti-tobacco posters etc. Can 0.05 ml of nicotine really kill a healthy 400-kg horse? What could be the origin of this phrase?

Toxicity: 30–60 mg (0.5–1.0 mg/kg) can be a lethal dosage for adult humans.

As a rough estimate, a horse can be 400kg, so a lethal does of 200mg-400mg for a horse. Density from wikipedia 1.01 g/cm³.

This means a lethal dose would take up 0.25-0.5 cm³ which is a drop of diameter 0.8-1 cm. This is quite a large drop, approximately 5-10times the dose you stated (0.05ml), but isn't too far from being a drop.

If horses are particularly susceptible to nicotine, then it could well be a lethal dose at 0.05ml.

Edit: This puts the LD50 (The dose where half the subjects die) in horses as 100-300 mg/animal, so if 2.5 0.05ml drops kill 50% of horses, its possible that 1 drop could kill a horse, even if it didn't kill most horses.