THEY may be considered the Viagra of the ocean, but a dose from the Tamar River could see you in hospital or even worse.
A new study has revealed Tamar's Pacific oysters contain up to four times the acceptable level of heavy metals.
The short-term effects of eating the toxic delicacies include nausea, headaches and vomiting.
Long-term exposure has been known to destroy the liver and even cause cancer.
As a result a renewed warning has gone out to estuary dwellers about the dangers of eating the wild shellfish.
Last year the Tamar Estuary and Esk Rivers Program investigated metal levels in oysters and recreational fish.
The fish were given the tick, but the oysters contained high levels of copper and zinc across the entire Tamar and cadmium in the estuary's upper reaches.
There has been a long-standing warning against eating the estuary's oysters, but the rivers program said the message wasn't getting out.
Program project officer Monique Thompson said oyster shells were routinely found at river campsites.
``The problem is there is no signage . . . particularly in very popular places that visitors will come and camp at like under the Batman Bridge,'' she said.
``That is where we are finding some of the highest levels of contaminants.''
Twelve signs will now be erected at popular river spots warning people off wild shellfish.
The metals are likely to come via run-off from urban roads and mining in the upper catchment and as a hangover from Launceston's industrial past.
Heavy metal levels in oysters, while unacceptably high, were lower than those in 2002.
Current programs are more geared towards managing the contamination rather than reducing it.