From Leopard Gecko Wiki
Supplements and Dusting
Leopard geckos require several types of supplements to their food in order for them to stay healthy: calcium to maintain strong bones, vitamin D3 to metabolize the calcium, vitamins to provide other trace nutrients that they require. There is concern among some people that leopard geckos can overdose on any of these supplements (especially vitamin D3 since nocturnal animals require only small amounts)while others feel that the gecko is able to self regulate appropriately.
There are a variety of different supplementation strategies that people use. Here are two of the most common and effective:
A. Keep a small container (milk cap size is good) of calcium without D3 in the enclosure. Alternate dusting feeders with calcium with D3 and vitamins.
B. Dust feeders at each feeding with Repashy Calcium plus (which contains calcium, vitamin D3 and other vitamins). Don't leave calcium in the enclosure.
As long as your geckos are getting regular doses of calcium, vitamin D3 and other vitamins, they will likely be properly supplemented.