What Causes Fainting And Incoherent Speech After Taking Zyrtec?

My husband has been taking zyrtec along with other prescribed meds yesterday he passed out was incoherent and had to be taken by ambulance to hospital. He did hit forehead and had a large lump c.t scan showed no concussion But Dr said he had overdosed on methadone he is 75 yrs old and never goes out by himself the Dr Dr said he tested positive negative for methadone was also combative. We care in shock as to what caused this and this diagnosis