A Lethal injection is used whenever the victim is needed to be executed or interrogated by an Interrogator to find the location of intelligence to the victim's affiliation and responsible faction to who's doing it.
The Soviet Union was the first and only faction in the Red Alert Universe to use a lethal injection. It was once used by an Interrogator who interrogates Tanya over where Professor Albert Einstein's location is and she refuses to give him the answer. He tries to inject her and the special ops agent fears that she will be killed by that. She kicks the Interrogator in the shoulder and he hits a table. He tries to inject her again, but gets distracted by a Spy. He instead kills the Spy and Tanya unties herself with getting the dead Spy's gun and shoots the Interrogator. She manages to get out and back to safety through the Soviet base.
In Command & Conquer: Tiberian Sun, Anton Slavik was captured by Hassan's loyalists and was placed on a chair being tied up. General Hassan sentences him to death on GDI espionage charges which was proven to be false. Hassan's medical doctor is ready to inject him with a lethal injection with Hassan saying it has a "100% toxin and no sedative" mixture. He tells the world that Slavik is being watched by the general public including GDI's military personnel such as James Solomon and Michael McNeil. The execution is running smoothly until Slavik is liberated by Black Hand loyalists and kills the captors inside. Slavik is rescued and brought to Kane where he asks him to be his second-in-command.