Is a crown a good idea after a root canal on a molar, or would a filling be adequate?

Crown is better: After a tooth has had a root canal, it loses some of its structural integrity, because the insides of the tooth have been hollowed from the drilling. As a result, the tooth needs to be reinforced to withstand the pressures of biting and chewing. A filling will fill the hole, but only a crown will cover the whole tooth surface, hold all the pieces together, and dramatically increase longevity.

Crown: After a tooth has a root canal completed, it can become more brittle, and the risk for fracture increases. A full-coverage crown, or an onlay, will help protect the tooth from fracture. In general, a filling is not enough, though there are exceptions, and your dentist would be the best person to help you make that decision.

Crown: It is highly recommended to place a crown on teeth after root canals, especially molars. After a root canal the tooth becomes brittle and a crown keeps the tooth from breaking. More importantly the crown provides a seal against bacteria and ensures that the root canal has a chance to heal and last a long time.

Depends: If the tooth was previously unrestored, and the root canal treatment was performed through a very small opening without sacrificing much tooth, then i would hesitate removing all that good remaining tooth structure to place a crown, that will ultimately need to be replaced due to decay, resultant gum pocket or material fracture. I would consider composite or bonded inlay/onlay.