Vaccine

Desmond injected this substance into his arm from a vial labeled "CR 4-81516-23 42" while he was living in The Swan. It is unknown how often he did this.
Ethan Rom injected Claire's baby with this substance while it was still in her womb several times.
- Ethan described the substance as a vaccine and told Claire there wasn't enough for both her and the baby.
- The vial in the Medical Station had the inscription "RX-1 GND" - the vial in The Swan only had "RX-1" printed on it.
The substance's identification number is also Hugo's winning lottery numbers.

Theories

This substance is a vaccine against the infection that spread throughout Danielle Rousseau's team.
It is a vaccine for the quarantine disease.
It gives The Others their super strength and super stealth.
The injections could just be a placebo to keep people scared. E.g. to keep Desmond thinking he has to stay in the Swan (just like the "Quarantine" signs) or to keep Claire in the Medical Station out of fear for her baby.
Could have GND in hospital because claire might have been administered Cocain to keep her calm and under control"
The injections grant or enhance psychic powers, allowing The Others to observe the castaways remotely, but betraying the observation with their backwards whispering; the reason Walt was able to "project" to Shannon was because The Others have been giving him injections (note, Walt spoke backwards on both occasions)
Isn't a chemical compound but contains immunity and physical characteristics enhancing nanotechnology in saline liquid.