Narasimha Rasayanam is a medicated butter  used in  ayurvedic medicine for the treatment of physical weakness and degenerative diseases.
It acts as whole body tonic and improves immunity and overall strength. It is also useful in Panchakarma Therapy (a detoxification and therapeutic procedure of ayurvedic science).
Benefits & Medicinal Uses:
The main benefit of Narasimha Rasayanam is to provide strength and improve immune system. 
It is general tonic, which improves overall health. 
It is beneficial for skin and improves skin complexion.
In ayurveda, it is also used as rejuvenative remedy. 
It is also helpful to improve hair growth and preventing premature graying of hair and hair loss.
Narasimha Rasayanam is prepared with the following ingredients:
Herbs:
Acacia catechu (Katha, Khair, Gayatri)
Dalbergia sissoo (Shisham, Shimshapa)
Embelia ribes (False Black Pepper, Vidanga, Vaividang)
Pterocarpus marsupium (Vijayasar, Asana)
Plumbago zeylanica (Cheeta, Chitrak)
Terminalia chebula (Hareetki)
Terminalia bellirica (Bibitaki)
Semecarpus anacardium (Bhilawa, Bhallatak) – Purified fruits
Liquids:
Water for preparing decoction of above herbs
Cow’s Milk
Elcipta alba (Bhringraj) – juice or decoction
Triphala decoction
Butter
Therapeutic Indications:
Narasimha Rasayanam is helpful in following health conditions.
General
General debility (but if has strong digestion powder)
Physical weakness
Muscular weakness or to increase muscular strength
For Weight Gain
Muscle, Bones & Joints
Muscle spasm
Low backache
Osteoarthritis
Dosage:
The dosage of Narasimha Rasayanam is as follows: 
People with weak digestive system should take it in a half tsp. once a day and slowing increase the dosage to 1 tsp. twice
a day.
People with good digestive can start from 5 ml a twice a day.
In Panchakarma therapy, you have to increase the dosage of Narasimha Rasayanam on daily basis as recommended by the Panchakarma therapist.
The safest therapeutic dosage of Narasimha Rasayanam is 2.5 ml to 10 ml twice a day.
Adjuvant:
People with weak digestive system should take it with warm water.
For rejuvenation benefits, you can take it with milk.
If you are willing to gain weight, then start taking it with warm water and if you can digest easily then start taking it with Ashwagandha Ksheer Pak (milk processed with Ashwagandha powder).