Lethium is a fast-acting drug that wipes out memory, causing an amnesiac state. The drug must be administered daily for optimal results.
Queen Azura drugs Flash Gordon with lethium, forcing him to forget his friends and fooling him into believing that she is his beloved. ("The Witch Queen of Mongo") Under the influence of the drug, Flash rejects Dale Arden, and leads Azura's army of Magic Men into battle against the Hawk Men.
Dale and Dr. Hans Zarkov plant the seeds of doubt in Flash's mind, and urge him to have Azura taste the drink laced with his daily dose. When Azura hesitates, Flash realizes that they're telling the truth, and he forces her to drink the lethium. Zarkov gives Flash medicine that restores his memory. ("The Witch Queen of Mongo")
On the eve of Flash and Dale's wedding, Princess Aura arranges to have Flash drink the "Draught of Forgetfulness". At Aura's urging, Flash rejects Dale, and fights Prince Barin. Zarkov and Barin manage to knock Flash out, and then put him in a machine that emits rays to counteract the effect of the drug. ("Chapter 110: The Unseen Peril)
Lethium, or the "Draught of Forgetfulness" as it is called, is also employed by the Forest People on Dale Arden in the serial "Flash Gordon's Trip to Mars". In this instance, the victim is commanded to take an oath to the god Kalu with the following words: "O mighty Kalu, I dedicate my life to your service. For it is written, that only by such a sacrifice can I atone for the wrong that had been done to you." After this ritual is observed the victim, under the hypnotic effects of the drug, will obey commands given him or her. Rendered submissive by Lethium, Dale Arden is instructed to take a consecrated knife and slay Flash Gordon himself.
Although Queen Azura comments that lethium is found only amongst the Forest People, Zarkov is familiar with it (possibly as a result of the earlier incident on Mongo), describing that "when burned, it gives off a vapor that deadens the mind and wipes out all memory of the past". He also knows that an antidote to counteract its effects can be prepared quite simply, by taking plain salt crystals, spraying them with barium and nitric acid, and treating them with heat.