Effects of nystatin cream?

Anti-fungal. Nystatin is an antifungal cream used for topical, skin yeast infections, such as athlete's foot, groin itch, etc. Side effects are minimal, but as with everything else, could get an allergic reaction to it.
The "effect" of . Nystatin is to treat fungus and yeast infections. I'm not sure if you're really asking about side effects, which are generally an allergic rash, which is very rare. All medicines have a risk that must be outweighed by the benefits. Sometimes, it's not an easy decision depending on the patient, the problem, and the treatment.