The principles of homeopathy mean that the more dilute a remedy, the more potent it becomes.
6c means that the remedy has been diluted and succussed (shaken vigorously) six times.
30c means that the remedy has been diluted and succussed thirty times.
As a general rule, for most acute and long-standing ailments, the 6c remedies are most suitable. For emergency and many chronic conditions, the 30c remedies are better.
However, it is more important to select the correct remedy than to use the right potency.
