Is it permissible to sell medicine boxes in the pharmacy at the old price, but at the new price after prices have changed?

If the state sets the price of medicine, it is not permissible to sell it for more than the determined price. If prices officially change, it is permissible to sell old stock at the new price, unless the pricing system in your country prohibits that.

Refraining from selling goods when the merchant knows of an upcoming price increase, in order to sell them at the new price, is considered injustice, fraud, and a forbidden monopoly.

Governmental pricing of essential goods is permissible and allowed if it meets the conditions of justice and prevents harm to people, such as when it applies to goods needed by the general public, and when the cause of the high prices is not a shortage of supply or an increase in demand; otherwise, it would be an injustice.