I have a tooth with large white fillings. my dentist suggests a crown to protect it just in case it may break. is is necessary?

Second opinion: If unsure, go for a second opinion. An onlay may be more conservative if needed, but you need to evaluate clinically and radiographically to make a long term sound decision.

When in doubt: if the recommended treatment is necessary, you should get a second opinion. You should always feel comfortable with your care, and treatment plan.

Lg white filling: Just because a tooth has a large filling, especially if it is a bonded white filling, does not necessarily mean it should have a crown.

Go with it: Your Dentist has probably determined that because of the size of the filling in your tooth the risk of fracture is high. In that case a crown that surrounds the tooth offers the best support.