Whey protein has been around for ages, and has a lot of research suggesting it is safe over the long and medium term. Additionally, it is made from natural sources, and whey has been an ingredient in many foods for a long time.
There are findings showing it helps improve exercise performance and encourages weigh-loss. It is no creatine as far as strength gains go, but there are so many more findings showing it is safe.
Weight Loss via Meal Replacement
Whey protein can reduce weight in people using it instead of meals. It is in fact linked to more weight-loss than using other protein sources according to one study – https://www.ncbi.nlm.nih.gov/pubmed/24724774
Strength Increases and Resistance Training
The same study shows a strong connection between using whey protein and achieving better results from resistance training.
One study shows 13kg extra in users 1 rep max on the leg press. That is quite a huge result, and based on a meta analysis of 22 studies and over 600 people. http://beefprotein.com.br/site/artigos/13.pdf
When to Consume for Best Strength Gains?
Quick answer – whenever. A study found it doesn’t matter when and all that matters is having enough protein in the first place to deal with the repair needed after training. https://jissn.biomedcentral.com/articles/10.1186/1550-2783-10-53
Reducing the effects of disease-caused muscle reduction
Whey protein has even improved patients’ strength and reduced risk of complications. It has medical use. http://www.sciencedirect.com/science/article/pii/S1568163711000900