Molecular Weight and Hair Growth

I saw the study about hair regrowth from a topical rapamycin shampoo, but I’m a bit skeptical of these findings.

The molecular weight of rapamycin is 914. At that weight, none of it will be absorbed topically. The hair follicles are 4-7mm below the scalp. If the rapamycin is not small enough to be absorbed, what is the hypothesis for how it can work on the follicle?

This same thing is an issue for topical dutasteride which has a molecular weight of 528, though it is suspected that some of it would be absorbed at that weight.

I’d love to hear from any doctors who have a perspective on how a topical rapamycin could work on hair.


I am not a medical professional but if you microneedle these in, there will be some penetration. I couldn’t say how much, but if you use 0.1-0.2 mm depth needle, you can do it daily. 0.5 mm, weekly, and 1 mm or deeper, monthly. I have a derminator but you can get a cheaper Dr. Pen.

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Does DMSO not affect absorbance?

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It does, by disrupting the skin lipid bilayer (ethanol, isopropanol, and surfactants do this, too), but I am not sure what the molecular weight limits might be for the compounds one wishes to push through. Edited to add, one could do both. If going deep with the microneedling, I’ll suggest stamping with it rather than rolling it because rolling can leave track marks.


Thanks, all.

I think what you are saying confirms my suspicion. Rapamycin is too large to be absorbed and get to the hair follicle, so it is unlikely to be helpful for hair regrowth topically. One could try microneedling to help absorption.

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I can’t say if it works for hair loss since that isn’t my issue. But it definitely improves the skin. Mine is radiant. I can’t stop looking at my skin!

I use a topical (91% isopropanol as the solvent) AM under sunscreen and PM under my tretinoin or tazarotene. I’ve also dermastamped once at 2.5 mm with it. I intend to continue at this depth monthly.

I’ll suggest trying it with three dermastamps (or one adjustable dermastamp) at 0.1 mm daily, 0.5 mm weekly, and 1.5 mm to 2 mm monthly. The arrector pili muscle insertion is around 1.65 mm depth.

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Never heard of a dermastamp. Which have you tried? Does it matter?

I guess you’re talking about something like this:

Alternatively, one could get a derminator or regular Dr. Pen and stamp with it instead of gliding it.

2 things - 1/ if absorption were an issue than face creams and wrinkle treatment would also stall, but there seems to be evidence to the contrary? i.e. works on wrinkles and fine lines 2/ what is the best technique for derma rolling a topical? Put it on then roll? Roll then put it on? Etc…

I wouldn’t trust that adjustable thing.

I have this one.

I’d get a few in different depths.

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Depends on what you define as evidence and the actives in question. For maximum efficacy in topicals, you focus on the ones that are small enough to get through the skin barrier.

If you use bigger molecules, you do need some kind of penetration enhancer(s). Ethanol, surfactants, + stamping in to create microchannels so that some of the actives get through.

If you use a dermaroller or dermastamp, put it on, then roll or stamp. I prefer stamping these days to avoid track marks. But rolling is probably ok for most people if they stay at a shallow setting (0.25 mm or less).

It can hurt if you use 1 mm or deeper, depending on the location on the face, so you might want to use a numbing cream first and use the guide below to adjust depth.

From Comprehensive Examination of Topographic Thickness of Skin in the Human Face | Aesthetic Surgery Journal | Oxford Academic
A comprehensive examination of topographic thickness of skin in the human face.
K. Chopra, 1 November 2015, Aesthetic surgery journal


I sterilize my face, apply numbing cream, apply a silicone reusable mask to help it penetrate, wait 25 minutes or so until my face is numb, wipe it off, apply my active(s), then stamp.

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Thank you. I have this one I have been using at 1.5mm on my scalp once a week. Electric Auto Dr. Pen A6 Microneedling Pen with Cartridges 12pcs 12pins &10pcs 36pins