But the scientists -- who included H. Fallah Huseini, PhD, of the Institute of Medicinal Plants in Tehran, Iran -- aren't recommending the extract to patients just yet.
Instead, they call for further studies to probe milk thistle's effects.
Their report appears online in Phytotherapy Research.
"The results are very encouraging, and now we need to do further large multi-centre studies," Huseini says in a news release from the journal's publisher.
Huseini and colleagues studied 51 adults in Iran with type 2 diabetes.
The patients were 40-60 years old (average age: 53-54) and had had diabetes for more than two years. Patients took oral diabetes drugs throughout the study.
The researchers gave the patients tablets to take three times a day with meals.
Half of the patients got tablets containing an extract made from milk thistle seeds. The others got placebo tablets containing no milk thistle or other medicine.
Patients didn't know which tablets contained the milk thistle seed extract.
At the study's start, the patients had similar blood sugar levels and blood sugar control.
The key might be compounds in milk thistle that are collectively called silymarin, the researchers say.
"We don't know the exact mechanism of action for this effect, but this work shows that silymarin could play an important role in treating type 2 diabetes," Huseini says in the news release.
No side effects were reported in this study.
However, milk thistle has been reported to occasionally cause a laxative effect, upset stomach, , bloating, and allergic reactions, according to the U.S. National Center for Complementary and Alternative Medicine (NCCAM).
The NCCAM recommends talking with your doctor before you take any herbal or other dietary supplements.