I have been taking tea tree oil internally (by mouth) for 5 months off and on. One week on, one week off. I have put 15 drops in a capsule, and swallow with a full glass of water. I got this from Rosemary Gladstar's book about herbs, and read that a woman tried this as a last resort when nothing else would work for her vaginal yeast infections. But I have not been able to find any info on any health risks of taking tea tree oil internally. I have also used it on someone that had a bad toenail fungal infection topically, and it did clear the nails in about two months. It has been the only thing that helped my irritable bowl syndrome (internally). I now believe that i had yeast throughout my whole body. I stay on a diet of fish and vegetables. Eat one kind of bread found at the health food store called Ezekiel 4/9 or 9/4. No dairy or sugar. No fruit except one tart Granny Smith apple a day, and Do a daily Colon Cleanse in a capsule form called Experience. I am drinking lots of water. I paid a Chinese herbal doctor a lot of money for this knowledge minus the tea tree oil. Also, Emu oil helps areas of sensitive skin that are raw from yeast especially the genital area. As for a vaginal yeast infection. I haven't had any until recently when I strayed from my diet. I'll use an herbal capsule from the Chinese herbal doc.
Although I haven't personally used tea tree oil, I understand that it's meant as an external treatment, and to be avoided as an internal med. Not sure precisely why, but a Google search should answer this easily enough. I've heard that as an external treatment it can be an effective antifungal. The links below (the first one is quite comprehensive) also mention external use only.