It’s possible that Elon Musk regrew his hair another way, but a hair transplant surgery is by far the most likely explanation.
First, established hair loss medications such as finasteride and minoxidil are unlikely to have regrown his hair so dramatically. In most cases, these treatments only stabilise and prevent any further hair loss. Plus, I don’t imagine he’s the kind of guy who’s in the mirror morning and night rubbing minoxidil into his scalp!
It could be a wig/hair system/toupee - whatever you want to call it. But he’s not been pictured without it as far as I’m aware and it seems like a lot of hassle for a busy entrepreneur who can afford a better solution - i.e. a hair transplant.
Hair transplant surgery is incredibly advanced nowadays - especially to someone of Elon’s wealth. There are a lot of misconceptions about hair transplant surgery - many dating back to early, less effective, procedures. It’s not ‘fake’ hair that’s used. The new hair is not ‘plugs’. Real hair follicles are simply moved from the back and sides of the scalp to the top. It then grows as normal hair would, only in a different place. What’s more, the FUE method leaves minimal scarring - even if hair in the donor area is cut incredibly short.
If you’re a good candidate for hair transplant surgery - and Elon appears to be based on those early photos - it’s possible to create an incredibly dense and natural appearance.
Elon - if he has had a hair transplant - is testament to how advanced today’s procedures are.
He’s got the money, the procedure is relatively straightforward and minimally invasive. He’s a problem solver, and hair transplantation is the best solution. Why wouldn’t he get a hair transplant?![1]
Footnotes
[1] Has ELON MUSK had a Hair Transplant? - Celeb Hair Transplants
Here’s Elon Musk, not at his best appearance:
And here is a pretty recent picture of Elon:
So, first thing first: Elon Musk has definitely done something with his hair.
According to various forums Elon Musk has had hair transplant at one point in his life. I do not seem to find any articles on his hair transplant or even hair change, which shows the fact that it was done secretly, and perhaps in a secretive clinic.
Wherever he has done it, they have done a good job.
Also, apparently, Elon has had a nose job, to reverse the effects of severe bullying and a broken nose when he was a child. Some people say that Mr. Musk has also undergone other cosmetic surgeries or procedures, yet none of these claims have been identified, and none are authentic.
However, kudos to Elon for working on putting away worries about his physical appearance, and focusing on more important things instead!

He most likely just had a hair transplant.
Public perception, so damaged by bad operations from the 70s, still hasn’t quite caught up to the state of the art in the modern era. Hair transplants are certainly not perfect and they can still fail. But generally speaking good results are possible if you’re willing to pay.
And you will have to pay a lot. Many transplant surgeons in the US charge up to $7.50 per graft, and to get results like Musk’s you might need 5,000+ grafts. That’s nearly a $40k operation.
So price alone is a big reason why you don’t see many men getting hair transplants.
The second non-monetary cost of a transplant is the recovery period. If you do a FUE operation, you’ll have to shave your head and won’t really be publicly presentable for a month at least. Not a lot of folks can get enough time off work in order to let the recovery process run its course. This goes for celebrities and high-profile actors as well.
Hair transplants. It “grew back” too fast and too thick (in the hairline) to be the work of minoxidil, propecia, etc., (which get thin re-growth at best).
He has to have hot, famous-actress women by his side to be the Alpha Male he needed (to force himself) to be to help sell his visions (he has many) to investors. And hot, famous-actress women are rarely going out with balding men (see point #6 below).
Seven reasons for men to shave their head (or go with a ¼” buzz cut):
[1] Rugs, drugs, or plugs = bringing an air rifle to a bazooka fight . . . . What’s the point of replacing one form of anxiety with another? . . . . What is the probability of success? . . . . What are the consequences of failure/disappointment?
[2] What is "Antifragility"? Read the book of the same name by Nassim Nicholas Taleb (©2012, Random House Trade Paperbacks).
"If you can't be a good example, then you'll probably have to settle for being a horrible warning." -- The Optimistic Pessimist
"Good judgment is usually the result of experience. And experience is frequently the result of bad judgment." -- Barry LePatner, testifying before a House subcommittee on science and technology (1982)
"Oh, yes. I've learned from my mistakes and I'm sure I could repeat them exactly." -- Peter Cook
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