An organisms that lives in or on another organism often with harmful side effects but without benefits is

This organism is a parasite.

Explanation:

It is called a parasite any organism that lives in association with another organism and establishes a parasitic relationship with this individual. Parasitism is a type of interspecific ecological relationship, that is, it occurs between individuals of different species, and establishes an association between two organisms in which one is benefited while the other is harmed.

This type of relationship is characterized by an individual, called a parasite, who lodges near or within the body of another organism, called a host. The parasite then steals nutrients, biomolecules and even uses the host's metabolic machinery to meet its physiological needs. Thus, the host, having part of its nutrients and biomolecules usurped by the parasite, becomes harmed in this relationship.

obviously, the latter.