Before using onion juice, you should be aware of a few things. 1. Onion juice can be made by blending and straining the chopped onions in a cheesecloth. 2. You can apply onion juice one to two times a week, depending on your desired results and hair type. 3. Start with two or three onions depending on the length of your hair. 4. Onion juice helps to boost blood circulation in the scalp, resulting in better hair growth. 5. It may cause irritation or a rash if used overnight, so discontinue use if that occurs.