Can I use a protein powder in shakes while on birth control? Will it decrease its effectiveness?

It's fine. Protein powder in no way effects the efficacy of your birth control. It's like wating a steak....It's just protein, not anything that interferes with your medicine. Make sure you drink plenty of water though, if you're doingprotein supplements so you don't overstress your kidneys.
No. It should not change effectiveness, but why not eat your protein as low fat meats, fish and legumes? Most people in developed countries are not protein deficient. Best wishes.