Warnings have been issued that driving a car or flying a plane can be risky after taking herbal medicines.
They came from the police and the Civil Aviation Authority.
In the latest issue of its magazine Vector, the authority said any pilot thinking of trying alternative medicines should consult an aviation medical examiner first.
The authority's senior medical officer Rajib Ghosh said people often thought a medicine which was called natural was completely safe and had no side effects.
"And that is not always the case," he said.
Products aimed at managing fatigue were an example.
"There are many natural treatments for fatigue on the market.
"Some even advertise that is is safe to operate machinery or fly an aircraft while you are on a course for these treatments.
"This is not necessarily true, which is why you should consult your medical examiner."
The police had similar worries.
Operations manager for road safety Peter McKennie said many substances contain ingredients that people were not aware of.
"And we are certainly not aware of the impacts of them. So it behoves anyone who is taking any of these substances to check to see what is in them.
"If in doubt they should ask a medical professional to make sure it will not have an impact on their ability to drive or operate machinery or even affect a medical condition," Inspector McKennie said.
The natural medicine industry disputes these arguments.
Patrick Fahy supplies alternative medicines and was a long time spokesman for the natural health industry.
He agreed it was important for people taking herbal remedies to check on what their contents were.
But they should not have to see a doctor to do this.
"There is a lot of information on the web regarding herbal medicines and their indications. And a lot of the information states when they should be used and when they should not be used.
"I think people should start using common sense instead of writing articles and saying people should have to see a medical doctor for everything - that is just nonsense," Mr Fahy said.