There is a popular aphorism in Russia, translated as "one drop of nicotine kills a horse":
, 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.
It takes more than a drop of nicotine, swallowed, to kill a horse.
This paper is about a number of mules that died from eating nicotine-contaminated feed. It claims, without a clear source, that:
The reported minimal lethal dose (MLD) of nicotine for the horse is 100-300 mg.
Tobacco has been used to de-worm horses, so it clearly isn't immediately toxic. It isn't recommended though.
Nicotine is an effective nerve poison - injecting it directly into the brain has far more severe effects, but that is hardly a useful warning against cigarette smoking.
Pharmacology and Toxicology of Nicotine with Special Reference to Species Variation Science 02 May 1958: Vol. 127, pages 1054-1055 reports actual experimental data by injecting nicotine into the muscles of horses.
It was determined that the "minimal effective dose" (meaning minimal dose required to cause paralysis) was 4.0 mg/kg and the "approximate lethal dose" was 8.8 mg/kg.
So a 400 kg horse would require more than a gram to cause paralysis and 3 to 4 grams to cause death, which is much more than 1 drop.