Biotin is commonly sold as doses approximating 1, 3 or 5mg capsules and there are no reported cases of adverse effects from taking high doses of the vitamin. Keep in mind that 5000 mcg is the same as 5mg and correspondingly 1 and 3 mg is the same as 1000 mcg and 3000 mcg respectively. Although Biotin is often recommended for strengthening hair and nails, the exact dose is not known. Taking extra doses of water soluble vitamins such as Biotin is not dangerous however your body will not be able to use the additional vitamin. You are very unlikely to get any benefit from taking a higher dose and the higher dose costs more without any additional benefit.