When can a player we have already replaced come back on?

An injured player who has been replaced may not return (Law 3.21). A player concussed or suspected concussed must be permanently removed (Law 3.24).

A tactically replaced player may return only to replace (Law 3.33):

• an injured front-row player
• a player with a blood injury
• a player with a head injury
• a player just injured by foul play, as verified by the match officials
• the player nominated to leave so a front-row replacement can come on (Laws 3.18 to 3.20)
• in elite rugby only, a teammate who received a qualifying (20-minute) red card, once 20 minutes of playing time have passed

Rolling replacements: a match organiser may allow rolling tactical replacements at defined levels, with no more than 12 interchanges; the organiser sets the rules (Law 3.34).