It is an excellent shampoo, actually the best for keeping dandruff at bay. But it does not have any active ingredient that directly or indirectly prevents hairloss.
What it DOES do, however, is keep your scalp is good shape. It should be a part of every balding man's regimen.
Remember, you can't fight hair loss with one weapon, you need to take a comprehensive approach and attack it with everything you have got.
Nizoral should be used twice a week for the first few weeks or less (depending on results) and eventually no more than once a week. You can also use a volumizing shampoo on one day, a DHT inhibitor on another two days and maybe yet another moisturizing shampoo on the final day of the week. By providing your scalp with a variety of care and stimulus, you will achieve a good all-round effect.
Some of the rigorous DHT inhibiting shampoos/treatments may help prevent hair loss, but they can also thin your hair and inflame your scalp.
Use a shampoo to clean, not to regrow hair. That's what Propecia/Proscar are for...
Max