Whether or not a bald patch can be made to grow hair depends on what has caused the baldness in the first place. If you are suffering from a fungal infection, and this is the reason for the development of the bald patch, then you can certainly grow your hair back. However, the focus of treatment should be on fighting the fungal infection rather than on growing hair. There isn't really such a thing as a hair tonic that can just be applied so that hair starts growing. If you have a fungal infection, you should first of all maintain proper hygiene. Wash your hair regularly with an anti fungal shampoo - for at least a couple of weeks, daily shampooing may be necessary, after which you can return to your regular frequency. In addition to this, at the end of every wash, after you have rinsed out the shampoo you should rinse your hair with a solution of vinegar and water. About a tablespoon of vinegar in a mug of water should be enough - do not use plain vinegar on a daily basis, as it will damage your hair.
You should also keep your hair and scalp dry at all times - if you wear a hat or a cap, wash it frequently so that the fungus does not build up inside it. Do not wear the hat or cap constantly, as it will accumulate moisture, and help the fungi to thrive. In fact, if for the next few weeks you can avoid situations in which you sweat a lot for more than a couple of minutes, you should. Whenever you do sweat, try to wipe your head with a towel, and dry your scalp as soon as possible. A fungal infection will most probably also cause itching, and if the itching does not subside within two weeks, then you will need to visit a doctor, as stronger treatment will probably be needed.
If the cause of your bald patch is simply age, then there is nothing you can do about it without visiting a doctor. Normal hair care, such as shampooing, conditioning, oiling, and protecting your hair from excessive wind and sunlight will marginally control hair fall, but these measures and home remedies cannot stop or reverse age related hair loss. Some modern drugs however are quite effective, and can not only stop, but reverse hair loss in some cases.
answered by M W