I started Valtrex (valacyclovir) for hsv1 the same time I got a Brazilian keratin treatment. I have had significant (loss of ~70%) hair since. Which cause this?

Valtrex (valacyclovir) and hairloss. I know of no known reports of Valtrex (valacyclovir) causing hair loss. As well, I do not think it would interact with your brazialian keratin treatment to cause this effect. I recommend you see a dermatrologist to better evaluate what going on and determine why you are loosing so much hair.
Very rare reaction. In clinical trials alopecia or hair loss was reported <1% but could not be definitely attributed to the medication. I doubt that the medication had anything to do with your loss of hair but anything is possible.