From Halopedia, the Halo wiki
Cytoprethaline is a cellular anti-crystalline drug designed for humans in cryo-sleep. Within the Unified Earth Government, cytoprethaline is an Rx-only drug; legal only through a medical prescription.
Cytoprethaline prevents damage to cell membranes from ice crystal formation. It is usually injected, recommended by a doctor, but usually by a medic or a pre-measured self-injector. The injection of cytoprethaline is routed into the subcutaneous tissue. Cytoprethaline was developed around the same time that cryonics became commonplace due to interstellar travel, to account for the dangers of low temperatures in cryo-sleep. Cryo-sleep is almost always fatal without the use of cytoprethaline. The only exceptions is SPARTAN-IIs where they can head straight to cryo-sleep without Cytoprethaline due to their augmentations.
However, 1 in 50,000 people exhibit an allergic reaction to this drug; those that do are disqualified from UNSC service or discharged if discovered after entering training, but can still serve in civilian roles attached to the UNSC. Thomas Lasky is one of the few people to be diagnosed with this allergy, which was to be grounds for his discharge from Corbulo Academy of Military Science; however, the school was attacked by the Covenant before Lasky was shipped out.
An allergic reaction to cytoprethaline results in blistering, peeling, or red skin rashes. Itching or hives, as well as swelling in the face, hands, or feet are other common allergic reactions. Swelling, or at least tingling, can also occur in throat or mouth. Trouble breathing, shortness of breath, chest tightness, cold sweat, and bluish-colored skin are common, severe results from a cytoprethaline allergic reaction. Sudden or severe headaches, or dizziness or problems with speech or with walking are other reactions.
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