The Borg Invasion that had made him a refugee so traumatised Sasdren that he attempted suicide by slitting his wrists with his claws. Sasdren did not die, and was taken in for treatment by the camp's Mark III EMH. As the EMH was programmed before the Federation had any significant contact with the S'ti'ach he improvised a treatment of polyadrenaline, which was progressively less effective as time went on. The EMH responded by continually upping the dosage.
When Doctor Beverly Crusher arrived on the camp she enacted a more effective treatment, overriding the EMH's judgment. Crusher used a delta-wave inducer and put Sasdren in a deep sleep, bypassing his subconscious mind, which had been resisting healing, and allowing his natural healing abilities to restore him.
Once Sasdren had recovered he stated he was better, which Miranda Kadohata's tricorder confirmed, and discharged himself. When Doctor Crusher learned of this she couldn't deny he physical injuries had successfully healed, but had doubts about his emotional wounds. (TNG novel: Losing the Peace)