Weeds in a drain

The trouble with pouring a chemical down the drain is that after you have a bath or water goes down it, it will be washed away and it will not have a chance to work on the weed. Also consider what damage will be done to the environment by pouring poisonous chemicals down the drain. If the weed is proud of the drain cover, I would suggest spraying boiling hot vinegar on the leaves and as far down as you can get, even pour some down the drain and avoid getting it wet for as long as possible.

Boiling water will help to see it off. And is eco friendly and cheap!

definitely, use boiling water, repeat after a week, it will work, esp. if it's cold the shock factor will add to the effectiveness

It also works on weeds in the cracks of the paving around the pond, then there is nothing toxic running off into the pond.