Disclaimer: Keep in mind I'm not doctor, I'm no scientist, I just read articles and observe.
It's a process called RNAi (Ribonucleic acid interference). In essence, it's a nanobot (man-made cell using a polymer and a real human cell), and it sends signals to cancer cells to basically kill themselves off (tested in labs, could potentially cure cancer). Now this works because it actually modifies specific genes (those genes that cause a cell to be cancerous). Now think about what this process is doing. It is "MODIFYING SPECIFIC GENES". Do you know what determines that your a man? Genes say to make XY chromosomes:
Now scientists have documented our genome, they've mapped individual's DNA structures, surely they know which switch in our genes causes production of XX or XY chromsomes? What's to say we don't create a few of these nanobot cells that think they are female cells, and you inject this into the body, and it starts telling the rest of your cells "Hey, why are you making male cells? Your supposed to be making female cells!". Well, then it's the game of "they'll tell two friends, and they'll tell two friends, and so on, and so on". It could be possible that your entire body's cells could be flipped to female ones within months, perhaps weeks? Your body would start producing it's own estrogen. Your body would start transforming on it's own (and "in theory" safely) because your body would regulate itself, no need for a doctor to monitor your dosages. "in theory", your body could grow a vagina since we're talking about more than just hormones, we're talking on the chromosome level.