A good fitting denture along with an adequate amount of saliva will usually allow a denture to fit well without the use of denture adhesives. However, over time, the gums will naturally shrink and thus the dentures will begin to loosen. The better thing to do is to go to your dentist for a reline. An impression is made of the new gums and new plastic is processed into place giving the denture a fit that is as good as new. In the interim, denture adhesive can be used, but it should not be relied on as ill-fitting dentures can be a source of chronic irritation that could possibly lead to malignancy. Another reason that denture adhesive may be needed is very flat gum ridges. If there is not enough bone present to resist the lateral forces of chewing, the denture could become continually dislodged.
There are a number of varieties of denture adhesives, mostly pastes, powders, and wafers. All types seem to work reasonably well, although pastes should only be used sparingly because they tend to be very thick in viscosity and could throw off the proper fit of the denture.