The Health Science Professionals Bargaining Association says the job action began with hospital pharmacists reducing their services to essential services only.
Starting Friday, the association says workers who deliver diagnostic imaging services will also cut back to essential services.
This group of workers include those who conduct x-rays, CT scans, MRIs, ultrasound, nuclear medicine and other imaging procedures.
The association says it also plans to reduce hospital lab services to essential services as of midnight Sunday, adding there will be no picket lines.
The association represents about 17,000 health science professionals.
"After nine months at the bargaining table, it took strike action and the involvement of mediator Vince Ready to get the government to come to the table and start to focus on getting to a new contract," said association president Reid Johnson.
"We will continue to exert pressure to encourage government to work towards achieving an agreement."