Conservation authorities and a veterinary team worked together to administer the injection, a tested contraceptive vaccine, to three females in the southeastern province of Trat. Another 15 females in other herds are in line.
The birth rate in Trat, as well as in four other provinces, is significantly higher than in the rest of Thailand.
This could create even more conflicts between humans and elephants in the long term if we let it continue, says Sukhee Boonsang, head of the country's wildlife management agency.
Nearly 200 people and a hundred elephants have died in such conflicts since 2012, according to official statistics.
The number of wild elephants in Thailand has increased from just over 330 in 2015 to nearly 800 animals - and several thousand more live in captivity.
