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There many websites selling biotin suplements claiming that they will make hair grow stronger and faster. Here are two examples:
Bio-info Hair Growth
Biotin deficency is shown to cause some problems including hair loss.
But a deficiency must be extremely rare: recommended doses vary up to 25mcg, which is easily gotten from an egg white. Supplements are generally sold with 3000-5000mcg per capsule.
Do biotin supplements increase hair growth and strength?
Biotin deficiency seems indeed to be rare. As with almost all vitamin supplementation, it's probably better to not take any unless you know you have a specific deficiency.
As expected, the sites you provide are full in anecdotal evidence and short on hard evidence, so stick with the Null Hypothesis until proven otherwise.
Very much recommended reading on the vitamin supplementation issue
