For those who study effects on human health, the main issue is not bovine growth hormone itself. Neither the natural nor the synthetic version is "bioactive" in humans.
The research has focused on another protein found in milk: insulin-like growth factor 1.
IGF-1 also is naturally present in the blood of cows and humans, in identical form, and at high levels it has been associated with certain human cancers.
And people who regularly drink milk have somewhat higher IGF-1 levels in their blood.
Yet these increases are very small compared to the natural range of human IGF-1 levels, said oncologist Michael Pollak, director of McGill University's Division of Cancer Prevention in Montreal.
As for a possible cancer link, he said, "there's no real smoking gun."
But a question remains:
In milk from cows treated with synthetic hormones, levels of IGF-1 can be even higher - by 25 to 70 percent, according to a 1999 review by a European Union scientific panel.
So does milk from hormone-treated cows - with the higher IGF-1 levels - have an even greater impact on the IGF-1 blood levels of people who drink it?
The answer is not simple.
Scientists disagree on whether any of that IGF-1 in milk survives the human digestive process and ends up in the bloodstream - or whether all the IGF-1 found in people is made by the human body.
In one study, researchers placed radioactive markers on IGF-1 and fed it to rats; they later detected the markers in the animals' blood. The amounts were even higher when the rats also were fed casein, a milk protein.
Mike Lormore, a doctor of veterinary medicine at Monsanto, was skeptical of that result. He said that the growth factor might have been digested and the markers continued into the blood on their own.
Janet Rich-Edwards, an epidemiologist and assistant professor at Harvard Medical School, called the matter "unsettled."
There is no dispute, however, that Posilac can affect the health of cows. The product label cautions that injected cows are at increased risk of mastitis and "may have reduced pregnancy rates."
A review by Health Canada - that country's equivalent of the FDA - estimated an 11 to 19 percent increase in mastitis, which is treated with antibiotics. Monsanto's Lormore said the increase was small when compared to other mastitis risks such as poor sanitation.
"Just like if you pick up some Tylenol, there's a label with everything known to man that can possibly ever go wrong with Tylenol," Lormore said. "The risk of those side effects is deemed to be very low and manageable."
As of now, the new Pennsylvania standards will take effect in February. And Harnish, the Lancaster County farmer, is happy with the change.
"There's something to be said for truth in labeling," Harnish said. "But I think we have to be real careful that the labeling does not imply something that's not true."
