A leading epidemiologist is worried the return of pseudoephedrine to pharmacy shelves this winter could lead to a return of a culture of people "soldiering on" at work or school when suffering with respiratory and infectious diseases like Covid-19.

University of Otago public health professor Michael Baker says pseudoephedrine masks Covid-19 and flu symptoms, meaning people could go back to work or school when they are still unwell and contagious.

Legislation reclassifying pseudoephedrine from a Class B to a Class C controlled drug passed its final reading this week.

Cold and flu medicines containing pseudoephedrine have been banned for 12 years over worries that the drug could be used to make methamphetamine.

Medsafe has approved 11 cold and flu medicines containing the medicine, Associate Health Minister David Seymour said on Friday.

Some pharmaceutical suppliers have indicated they may be able to supply the first products in June.

Baker says when cold and flu medicines containing pseudoephedrine were last available they were marketed "as a way to manage your symptoms so that you could 'soldier on'".

"It was the idea that it was good to get back to work. It was part of our culture - we weren't going to let a respiratory infection get in our way of going to school and all these other things.

"Now we've learned that it's actually a very bad idea, it's antisocial, to go to work, school, and social events, and infect people around you.

"Hopefully that's a lesson that is now very well-established."

Baker says pseudoephedrine also masks many of the symptoms of Covid-19.

"Not only would people potentially not recognise them as easily as being severe if they started taking them early on in their illness, but they also might feel pretty comfortable going back to work and school and so on, with these symptoms, when in fact they are still very infectious to those around them.

"I think that's a potential problem with making pseudoephedrine more available than it is present."

Baker says it's natural for those suffering severe colds and flu to want the best treatment of symptoms, but the government needs to consider how the medicine fits in with other approaches for managing respiratory illnesses.

Michael Baker. Photo: RNZ.

"We all feel miserable [with those infections] and you want to grab the strongest thing you can find to relieve those symptoms. But we have to balance that against these potentially unintended consequences of people who are really quite sick feeling compelled to go to work or school."

He says not everyone can work from home, and those people "may feel quite compelled to take those medicines even when they're actually quite sick and go and infect people around them".

Baker says respiratory and infectious diseases like Covid-19 can cause long-term harm and seriously affect productivity.

He says the government needs to make sure it has a comprehensive respiratory and infectious disease strategy that included encouraging vaccinations, taking rapid antigen tests (RAT) and self-isolating, to support people getting back to work and school "as soon as they can, and as soon as they can do it safely".

"Obviously it's great to have good symptomatic treatment but you don't want people soldiering on as they did in the past, filling themselves up with pills and going to work and school because they have to.

"Some people may feel very pressured to go back to work if they don't have good sick leave entitlements, or they might feel very compelled to send their kids to school if they're going to be fined for absenteeism without what might be regarded as a good reason.

"We have to make sure that illness is considered a good reason for children not going to school."

-RNZ.