Yes, Milfoil is toxic to dogs!
Dogs accidentally consuming the Milfoil plants can show the following clinical symptoms: Vomiting, Prolonged Depression, Incoordination, Hypersalivaton, Sleepiness Or Excitation, Dilated Pupils, Low Body Temperature, Low Blood Pressure, Coma, Seizure And Death (In Rare Cases).
Milfoil consumption is also toxic to Cats and Horses.
Toxic Principles Of Milfoil: Glycoalkaloids (achillein), monoterpenes, sesquiterpene lactones.
More Info About The Milfoil Plant
Scientific Name: Achillea millefolium
Family: Asteraceae.
Also known as: Yarrow.
The answer is based on information found on The American Society for the Prevention of Cruelty to Animals (ASPCA) website.