There are a variety of synonyms for the phrase "took effect", which means something became valid or began to work. Some options include "came into force", "took hold", "went into operation", "became active", "commenced", "initiated", "came to be", "came into operation", and "enacted". These terms can be used in legal, medical, or general contexts to describe the start of a new situation, such as a law being implemented or a medication beginning to have an effect. Choosing the appropriate synonym can add precision and clarity to your writing, helping readers to better understand the context of the situation at hand.