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The "Unknown Medicine" in this series is the medicine that several characters try to inject into one another. In reality, this is C-120 (an alternate version of it is named C-103), a treatment for Hinamizawa Syndrome. It usually appears as a syringe full of golden, orange or colorless liquid. The vaccine version of the Unknown Medicine is called C-117.
The medicine was developed by Irie Kyōsuke and is designed to suppress the symptoms occurring in the Syndrome's LV3 to LV5 stages by decreasing epinephrine, norepinephrine, and cortisol hormone levels. Epinephrine, cortisol and norepinephrine are hormones secreted by the adrenal (adrenaline) gland under increased anxiety and stress. Together, they can induce psychotic paranoia responses and act as stimulators that make the Hinamizawa Syndrome pathogen incredibly unstable; so, by decreasing these hormonal levels, the mood and adrenaline energy of the victim will be depressed and the pathogen will remain dormant. Since the hormones are naturally secreted by the adrenal gland, the C series injections are only short-term. In Matsuribayashi-hen, Irie and Takano Miyo report that since the C-120's purpose is to depress hormone levels, anyone living normally under LV1 or LV2 (or even don't possess the Syndrome) injected with the C-120 will have their stamina levels drastically reduced and experience mimicked LV5 symptoms such as lymph node rashes, muscle spasms, and headaches.
In Onikakushi-hen, Keiichi hallucinates Mion and Rena attempting to inject him with a strange medicine. In reality, they were trying to draw on his face and shirt with a marker, and the syringe that Keiichi sees is actually said marker. Unfortunately, Keiichi does not recognize this, and beats the two girls to death with Satoshi's baseball bat.
In Meakashi-hen, Rika attempts to stop Shion by injecting her with C-120. Shion doesn't know what the syringe is filled with, and also takes this as a death threat. The two girls fight, and Shion injects Rika with the medicine. Rika has a bad reaction to C-120, and ends up losing a large chunk of her stamina. Shion wants to see Rika die from the medicine, then decides to scratch that idea and take her to the torture chamber. However, Rika remembers being tortured by her in Watanagashi-hen, so she takes a knife and commits suicide in order to escape that fate.
In Watanagashi-hen, Rika's body is found with the cracked syringe in her pocket, the contents having leaked out before discovery. Without the foreknowledge she had in Meakashi-hen, it is most likely that she was subdued or killed before having a chance to employ it.
While Rena is hiding out in her personal van in the dam construction site, Rika comes up with a syringe case, scaring her. Like Shion in Meakashi-hen, Rena does not know what the syringe does. Thus, she immediately determines that Rika is an 'alien copy', based on her theory. Although Rika gives Rena the syringe and medicine, she does not force it on her since she remembers what happened in Meakashi-hen. In the visual novel, Rena states that "It (Rika) closes the case of the syringe, and puts it back into its pocket," so it's almost certain that she never took the shot, thus leading her to continue progressing to LV5 Hinamizawa Syndrome.
In Minagoroshi-hen (and presumably all scenarios before this), H-173 is used by Takano Miyo to drive Tomitake Jirō into LV5 symptoms (although Tomitake believes himself to be vaccinated against Hinamizawa Syndrome, Takano had switched his vaccine for a placebo without his knowledge), causing him to claw his throat out after being released.
The name "H-173" is a Japanese pun akin to that of Ryukishi07's own pen name; the numbers can be pronounced as "hi-na-mi", as in Hinamizawa Syndrome.