Yes, they may be deductible if you itemize and the total expenses for you, your spouse, and your dependents exceed 10% of your adjusted gross income (7.5% if you're 65 or older as of December 31, 2016). The deduction is limited to the expenses above and beyond this amount.
For example, if you're 48 years old with an AGI of $50,000 and you incurred $5,900 in medical and dental expenses last year, you could deduct $900 (the amount in excess of 10% of your AGI, or $5,000), assuming you're itemizing.
You won't be able to deduct your expenses on your federal return if you're taking the standard deduction, which about 2/3 of all taxpayers do.
However, don't let that stop you from entering all your medical-, dental-, and vision-related expenses anyway. Some states will still let you deduct these expenses even if you weren't able to deduct them on your federal return. The expenses must not have been reimbursed (paid back) by your insurance company.
Note: You can reimburse yourself from your HSA but those would not be deductible because the money in your HSA is pretax. See Can I deduct medical costs paid through my HSA or MSA?
