As long as there is some substitution effect in response to a change in the relative price of a good,there will be an excess burden from a tax.

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The excess burden of a tax, also known as the deadweight loss, is the loss of economic efficiency that occurs when the tax causes buyers and sellers to change their behavior in ways that reduce the quantity of goods and services exchanged. The substitution effect of a tax refers to the fact that a tax on a good will increase its relative price compared to other goods, thereby making buyers substitute away from it towards other goods that are relatively cheaper. This substitution effect creates a distortion in the market that reduces economic efficiency and leads to deadweight loss. Therefore, as long as there is some substitution effect in response to a change in the relative price of a good, there will be an excess burden from a tax.

As Long as There Is Some Substitution Effect in Response