i'm confused but i think he drank the antidote, so he just threw up because poison is leaving his body. but i'm not sure how will they connect her dying in the past and returning the modern time because it doesn't make sense.
he comitted su*cide after ji won died. he found out that she hospitalized because of cancer, wanted to visit her but learned that she had left the hospital. then he visited her house but sadly he saw her dead body.