Such buyers were putting their health at risk by bypassing official regulations aimed at ensuring the safety, quality and effectiveness of prescription medicines, Prof Norris said.
"I think that people shouldn't buy their medicine over the internet.
"You just can't be sure what you're going to get. It's very risky and there's no-one to complain to if something goes wrong."
Access to drugs via the internet potentially meant "we're going back to the bad old days when anyone could sell you anything" and there was no guarantee of product safety, she said.
The two academics commented yesterday after Otago University fourth-year pharmacy students Coran Turner, Gerard Casey and Stacey Pigou - and staff and students from the Auckland University School of Pharmacy - analysed samples.
The medicines had been intercepted at the border by staff from Medsafe, the Ministry of Health unit which regulates therapeutic products.
The students looked at two of the most frequently bought medicines on the internet - tadalafil (for erectile dysfunction) and sibutramine (for weight loss).
Some tablets were misshapen and others were mouldy.
Dr Strachan said it was "particularly concerning" some tablets looked genuine but were counterfeit and contained only 40% of the stated amount of active ingredient.
People often did not realise the risks involved.
In some cases, drugs were "manufactured in dirty conditions where there is little or no quality control" and the labels could be highly inaccurate.
"Other dangers include possible contamination with bacteria, mould or other foreign matter such as brick dust, arsenic or lead," she said.
