Brief Answer:
avoid direct sunlight, use of antioxidants, elemental zinc and copper.

Detailed Answer:
Hello and thanks for your query.

I shall make an effort to provide you with good professional recommendations specific to your questions.

The premature hair gray is nothing but a process where the pigment is not being produced from the cells which produces color.
Genetics, vitamins, habits of smoking and lack of proper diet are responsible for grey hair causes.
Vitamin and minerals, especially vitamin B12, zinc and copper are responsible for giving hair its natural color and deficiency of these vitamins and minerals affects the hair color. Smoking and diet especially junk foods or processed food accelerate aging by increasing the oxidative stress, results in loss of hair colors.

I would have suggested to avoid direct sunlight, use of antioxidants, elemental zinc and copper will help to prevent the premature hair graying and apart from that avoid smoking and also avoid stress, try to be relaxed to prevent premature graying of hair.
Medications are not available to reverse the process, it can be prevented.The only way to turn your hair into black colored is using XXXXXXX or by regular use of mehandi may have safe effect.

Yet again, I duly appreciate your query to me, I do hope that you have found something useful to help you. Please accept the answer if you have no follow up queries.
Wish you good health.

Sincerely

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