Wed Nazir 60

The gemara inquires if a Nazir Tamai needs to shave his hair with a razor or even by putting on some potion that will make his hair fall out.

Tosfos point out that this refers to when he shaved part of his hair and the rest with the potion. We would never have though he's allowed to do all his hair with a potion, since the Gemara in BK says this potion will cause the hair never to regrow, if he would put the potion on his hall hair, then he won't have hair to shave for when he becomes a Nazir Tahor.