How much glucomannan for weight loss?

A common approach is 3g per day in three 1g doses before meals.

Do I need to drink a lot of water with it?

Yes — at least a full glass of water per dose, otherwise there's a choking risk.

Gas in the first week, then it tends to settle down. Avoid if you have swallowing difficulties.

Glucomannan: A Well-Studied Fiber for Weight Loss

Among all the ingredients marketed for weight loss, glucomannan stands out as one of the most extensively studied for its role in supporting a calorie-reduced diet.