I believe temporary fillers are excellent with very few side effects. Permanent fillers are more prone to give side effects such as granuloma formation, fibrosis or scarring. I do not use synthetic permanent fillers. There are a number of popular dermal fillers. One does not need to have an allergy test prior to using dermal fillers.
I believe the best permanent filler is your own fat. Fat injections can be used to augment lips, and soften naso-labial folds as well as filling out hollow faces. Fat injections can similarly be used to fix hollow areas around the body from accidents, surgery or congenital or acquired deformities.
Fat injections are a very good technique. Approximately 50% of the fat will dissipate and so overcorrection is desirable.