How does Ashwagandha affect body weight? 
Dr JV Hebbar
Ashwagandha is a unique herb which can be used both for losing and gaining weight.
For weight loss: 
For weight loss, it is administered in capsule or powder form along with honey and water. 1 - 2 grams of Ashwagandha powder with a teaspoon of honey, swallowed with a cup of cold water, once a day, after food.
It improves muscle strength, hence enables one to be more physically active, hence useful in weight loss. It also helps to relieve depression. Many depression patients often put on weight either due to depression itself or due to modern anti depressant medication. In either case, Ashwagandha is useful to fight both depression and related weight gain.
Read: Effective Obesity Treatment, Remedies, Ayurveda Tips
For weight gain
Because it improves muscle strength and has immense nutritive value, when it is used along with other high nutrition foods like large quantities of ghee, milk and sugar candy, Withania somnifera is very useful to put on weight.
For weight gain, 1 teaspoon of Ashwgandha is usually consumed along with a teaspoon of ghee, mixed and swallowed with a cup of lukewarm cow milk or water. 
Ashwgandha, being hot in nature, improves digestion strength and hunger, thus enabling person to consume more nutrition rich food.
For weight gain, higher doses of Ashwagandha avaleha (herbal jam), Ashwagandha ghrita (herbal ghee) are useful. The dose should be more than 1 - 2 tablespoon per day. 
In lower doses, they do not contribute much to gain weight.
Read: Healthy tips To Put On Weight, To Become Fat Naturally

Wish you good health :) 
Dr JV Hebbar