Jealousy can effect a relationship in a negative way. The person in the relationship who is jealous will never feel at ease and will have an issue with trust. While the other individual in the relationship will always be on edge.
A little jealousy can be healthy in a relationship. However, if one person is excessively jealous, then it can poison the relationship. Trust is everything in a relationship.
Over time jealousy will definitely affect a realtionship. How can someone be jealous constantly and expect their partner to deal with it. Sometimes the jealousy is a cover up for something that that person has done.