Dodgy pills and medicines are on the rise, as more people buy prescription drugs online.

Medsafe and Customs have been involved in an international initiative called Operation PANGEA, which tackles the illicit trade in medicines.

It's led to Customs stopping 426 packages for investigation, more than twice as many as last year.

Compliance manager at Medsafe Derek Fitzgerald says when it comes to prescription drugs - the best thing is to buy them in New Zealand.

"It's important that if you have a condition where you need a prescription medicine, you should see your doctor in New Zealand and have it prescribed for you and obtain that through the New Zealand distribution chain."

Fitzgerald said there's always a question mark about the quality of medicines being bought from overseas.

"They could be counterfeit, they could be unacceptable in some way, be poor quality, they may be contaminated. "