18 weeks (19 on Friday) nausea/vomiting

Hey ladies! Question... This is baby number four for us and with previous pregnancies nausea/ vomiting ended around week 12 or shortly after. This time is started at the end of week 14 and has gotten progressively worse. Doctor gave me medicine (promethazine) at last visit but I can't even keep them down for longer than 20 minutes. She also told me that when developing morning sickness later than the normal time time frame it can last through the remainder of pregnancy. Anyone else go through this? Or have any advice? Thanks!