Here’s an excerpt from the Taloustaito article linked below regarding the taxation of capital repayments before and after 2014. The entire article is worth reading.
"CAPITAL REPAYMENTS RECEIVED BEFORE 2014
When a shareholder received a capital repayment, they did not pay tax on it. The time for tax payment comes when they sell the share. “At that point, the acquisition cost of the share decreases by the amount of the capital repayment. When the acquisition cost decreases, the taxable capital gain and therefore also the tax on the gain increases,” Juha-Pekka Huovinen explains.
CAPITAL REPAYMENTS RECEIVED FROM THE BEGINNING OF 2014
If a shareholder receives capital repayments from a listed company, they are taxed in the same way as dividends from a listed company. Withholding tax is also applied to them, just like with dividends. Capital repayments received after the beginning of 2014 do not affect the acquisition cost of the share or the calculation of taxable gain or loss on the sold share."