Methotrexate is an antimetabolite used to treat certain types of cancer.
A lab in Wolverhampton is suspended as 43,000 people in England and Wales are potentially affected.
Some people who took tests in Berkshire earlier this month may have wrongly been given the all-clear.
It comes as ministers unveil £250m package for GPs to encourage more face-to-face consultations.
Other groups have reported higher protection, but the results are still seen as a triumph.
A question everyone's asking: what's my Covid risk? Here's how to find out.
Borrowing - which hit a record £22.3bn in October - cannot go on indefinitely, chancellor says.