Ordinary income tax plus a 20% penalty if you are under 65. After 65 the penalty disappears and only income tax applies.
Under 65, a non-qualified distribution is hit twice:
On a $5,000 unsupported withdrawal in the 24% bracket, that's $1,200 in income tax plus $1,000 in penalty — $2,200 gone from a $5,000 withdrawal.
The 20% figure is worth noting. It's double the 10% early-withdrawal penalty on retirement accounts, and it applies at any age below 65.
At 65 and older, the 20% penalty disappears entirely. Non-qualified withdrawals are simply taxed as ordinary income, which makes an HSA behave like a traditional IRA — with the upside that qualified medical withdrawals stay completely tax-free.
Note what this means for missing documentation: a distribution you were entitled to make becomes penalized if you can't prove it. The tax isn't punishing the withdrawal. It's punishing the missing paperwork.