There is, I think, serious reason to suspect that she may not be.
In July of 2014, the senator announced she had Stage IV lung cancer but that it was not spreading. According to her, her doctors former DSWD Secretary Esperanza Cabral and Ruth Divinagracia had recommended she take pills for "molecular targeting" of cancer cells. Now, Senator Santiago claims she had "licked cancer" and is now ready to run for president.
This is troubling because Senator Santiago's account of her diagnosis and her now buoyant pronouncement of being finally cured of cancer do not quite add up.
First, Stage 1V cancer precisely means that the cancer has spread from the original site (see this information from Cancer Research UK on lung cancer number staging system). The senator's previous claim that she had Stage IV cancer but that it had not spread was, therefore, oxymoronic and did not make any medical sense. A lung cancer is declared Stage IV precisely because it had spread.
Second, Senator Santiago was probably taking Tarceva (erlotinib) tablets in lieu of chemotherapy as her first-line treatment. Perusing the prescription sheet of Tarceva, you don't get the sense it can magically cure stage IV lung cancer. And cancer could always come back from remission. It would be too early to say that Senator Santiago is in the clear now.
What does this all mean? The greatest issue that will hound Senator Santiago's candidacy now will be the same issue that hounded her candidacy in 1992 -- the state of her health. The ruckus about her health in 1992 was probably baseless, fueled by malicious rumour-mongering, but this time it could be quite legitimate.