Hi greetings. Reversing grey hair to black is indeed among the tough results to show to a patient since this is a multifactorial problem. Since in most it's genetically mediated, it's tough to reverse or treat. However there are certain peptides and newer molecules application of which may be worth a trial. It's also important to rule out hormonal irregularities and nutritional deficiencies. Increase intake of oral anti oxidants. Meet a dermatologist near you for further advise. Using henna is safe - provided its pure henna and that you are not allergic to it. Most ' black mehendi' / 'black heena' have PPD like chemicals which can cause llergic reactions in those susceptible, so be cautious. Hope this helps. Dr Govind Suresh Mittal.
PS - Further queries can be sent to follow up messaging.