The 33-year-old Jantjies said in August he tested positive for Clenbuterol, which can be used to burn fat, and would fight to prove his innocence.
The South African Institute for Drug-Free Sport (SAIDS) said on its website that Jantjies has been given the ban, which might bring an end to his career.
Jantjies, a flyhalf, was a member of the squad that won a second Rugby World Cup for the Boks in Japan in 2019. They retained the title in France last year.
Jantjies was picked in South Africa’s enlarged training group ahead of the 2023 World Cup but didn’t make the final squad.
SAIDS said Jantjies tested positive for Clenbuterol in June last year and that no hearing was held about the case.
Jantjies’ last club was Agen in France.
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