There are prescription-only high-fluoride toothpastes. Excessive fluoride (more than you can get from Act + normal fluoride toothpaste) can cause fluorosis, which ends up being bad for the teeth. Normal amounts of fluoride are good for the teeth. The prescription-only toothpastes have enough that you don't want to use them regularly for a long period. As with most things, the dose makes the poison. Some dentists prescribe them even when they're not needed, since it won't be harmful to use a tube of the stuff every year, or even every 6 months, and it's profitable to them.