My comment: The needle length should be 1 -= 1.5 mm long to reach the growth center of the hair follicle. Read this article: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3746236/ The question raised is how often does the dermaroller (microneedling) need to be done and for how long? The article doesn’t really answer this question but does show the superiority of using microneedling with minoxidil rather than minoxidil alone.
Comment by Reader: How long should I wait before applying to my scalp? I use the 0.5 mm roller on my hairline, and I use rosemary/lavender oil instead of minoxidil, how long should I wait before applying this? I’ve done it as soon as I finished rolling, and it stung but I’m not sure if this is good or bad.