HSA (health savings account)
A health savings account you can also invest.
An HSA is a US account for medical costs, available if you have a high-deductible health plan. Many HSAs let you invest the balance.
The triple tax advantage
Contributions are tax-deductible, growth is tax-free, and withdrawals for qualified medical expenses are tax-free. After age 65 you can also withdraw for any reason, paying regular income tax on non-medical use, much like a traditional IRA.
The long-game approach
Some people pay current medical bills out of pocket, invest the HSA, and let it grow for decades as an extra retirement account.