Romeo took the poison not knowing that Juliet wasn’t actually dead, but in a state that appeared like death due to drinking the potion that Friar Laurence gave her, because he never got the message that Friar John was carrying due to Friar John being quarantined after crossing an area where there was plague. However, Romeo’s servant Balthasar did manage to get to him, only he merely passed on the news of her death and was unaware of Friar Laurence’s plan with the potion. That’s why Romeo went to the apothecary to buy poison. He didn’t want to live if Juliet was dead. Juliet wakes up after he’s taken the poison but before he dies. Once he’s gone, she realizes there isn’t enough poison left over to kill her too, because she also doesn’t want to live if Romeo is dead, so she uses the only thing at hand guaranteed to do the deed—-Romeo’s dagger, which he had just used to kill Tybalt, Juliet’s cousin. (Yet people think Shakespeare’s plays are boring. Cripes.)

He appears to be dead and in fact really is!