Deltanine

The subject was a 14-year-old male Indian elephant named Tusko being housed at the Lincoln Park Zoo. As previous research had suggested that high doses to LSD were needed to get perceivable effects in "lower animals," they decided to start with a 0.1 mg/kg dose of LSD for Tusko. That came to about 297 milligrams (in 5 mL of water, injected intramuscularly) of LSD for 7000 pound Tusko. The injection was delivered via a pressurized CO2 dart gun. For comparison, the threshold dosage for an effect in people is around 20-30 micrograms and a recreational 3+ hour dose would be around 100-200 micrograms.

After injection:

"Tusko began trumpeting and rushing around the pen, a reaction not unlike the one he had shown the day before (during the placebo shot). However, this time his restlessness appeared to increase for 3 minutes after the injection; then he stopped running and showed signs of marked incoordination. He began to sway, his hindquarters buckled, and it became increasingly difficult for him to maintain himself upright. Five minutes after the injection he trumpeted, collapsed, fell heavily on his right side, defecated, and went into status epilepticus."

The researchers were understandable quite surprised and distressed by the elephants seriously bad reaction to the LSD (they theorized that elephants were actually quite resistant to psychotropic substances). They tried to alleviate the seizures by administering promazine hydrochloride (Thorazine, 2800 mg!), which had a modest effect in reducing the spasms but did nothing for Tusko's overall state. A last ditch effort to save the animal by injection of pentobarbitol sodium was also unsuccessful, and Tusko died 1 hour and 40 minutes after the LSD dose.

Read more about it here