The Capitol used the poison to alter Peetas memories involving Katniss and turn him against the girl he has been in love with his whole life. The Rebellion took action and started trying to hijack him back using a similar method. Peeta never returned to the way he used to be exactly for a long time, the brainwashing had lasting effects on him. In the end both sides of the war used a form of brainwashing to get Peeta to act the way they wanted. Brainwashing has been tried many times in experiments by many agencies including the CIA based on different drugs and methods.
The tests were never very successful but as science progresses there is a possibility that brainwashing could become a reality. Mockingjay portrays that the brainwashing was done by altering the memories of the affected. It was based with a genetically engineered poison that acted as a hallucinogen which allowed the capitol to tamper with the memories that the patient was recalling. This method made the brainwashed person act exactly as intended because their memory was altered. Using a similar method governments could use this is a multitude of ways, good and bad.
The ethics involved with the practice of brainwashing would be very hard to justify, completely changing the way a person thinks and acts isnt ethical. The main idea behind brainwashing is harmless enough, it can be as simple as getting everyone to agree with you all the time. Maybe even making everyone think you accomplished a feat no one thought was possible. More seriously, it could even make people completely turn their back on a lifelong friend. Brainwashing is harmless until it gets taken out of control ith a bad motive behind it.
For example, controlling a president to run their country into the ground and create civil war could be considered unethical. I think that because of how uncontrollable the human race is, working on a way to brainwash people is undoubtedly unethical. There is some usefulness to the practice of brainwashing but it would be almost impossible to not let it get into the wrong hands. It could be used to get information out of spies and would force security to be tighter.
It would also be useful to rehabilitate convicted felons to help them get their life back on track. Also, it could help addicts to help get over their addiction and move on in life. I think the topic of weather to use it or not would be a big debate in countries. Brainwashing can be helpful but also harmful to those involved and the question of the ethics of it is up for debate, but personally I think it is an unethical practice because of how uncontrollable humans can be.