Xylazine, a sedative and tranquilizer commonly used in equine practice, is safe and dependable in horses compared to some other drugs. A concern is that it can be deadly in humans, causing very low blood pressure and low heart rate, depending on the amount of drug ingested or injected. Veterinarians are always careful about dispensing xylazine for horse owners to have in their barns or emergency kits. 

A Philadelphia study found this tranquilizer present in nearly one-third of fatal opioid overdose cases in that city. Using xylazine with opioids increases the chance of overdose and death. It is believed that adding xylazine to the illegal drug combinations used by humans make the effects of the drug last longer.  

Dormosedan is in the same drug class as xylazine and is also deadly in humans. Abuse of these drugs can result in them becoming classified as controlled substances, increasing their price and limiting their use. 

You should not store these drugs in the barn or unlocked vehicle. Instead, lock them up since they can be deadly to humans when misused.