Immediately follow with 1/2 cup of white vinegar . Cover the drain opening with a plug or a damp cloth to force the fizzing action downward into the pipe.

Pour 2 quarts (about 2 liters) of boiling water down the drain to flush away the dissolved material.

Remove standing water from the sink or tub as much as possible. (Use a cup or bucket to bail it out if necessary.)

An Effective Homemade Drain Unblocker: A Chemical-Safe Alternative