I got headaches. Not lingering throbbing ones, it was different. For example, if I put it on my scalp before bed and then tied my hair up as I normally do, the scarf felt too tight and my head would start to hurt - you know how it feels when you tie the scarf on too tight. Also I would get a head rush kinda headache. Like while I was doing my regular Pilates work and had to hold my breath or strain or something, I would get a head rush. I sometimes woke up with headaches, too.
This decreased when I stopped using it everyday. I started using 2 times a week instead of daily and these side effects stopped for the most part.
Then I stopped using MN all together and I feel totally normal. I have not been using the MN now because I don't want to be bothered anymore. I keep wanting to because it does CRANK out some hair, but it became more of a nuisance for me.
It also can cause dryness of the hair, but that can be solved my adding it to products or using the moisturizing MN products that some ladies use.
Other than that I never noticed anything. But, headaches are enough!