According to the Mayo Clinic, whey protein supplements are likely safe for most adults, but whey protein should be avoided by those who are lactose-intolerant or have a dairy allergy. Whey protein might lower blood sugar, so diabetics should seek medical advice before taking it.
Whey protein can increase bleeding, so it should not be taken by those with blood-clotting disorders. It may also interfere with heart rhythms and cholesterol levels and cause intestinal distress, such as bloating and constipation. As of 2014, there is no scientific evidence on the safety of whey protein for those who are pregnant or nursing.