Dr. Brown & BPA Free Nipples reduce colic, vomiting, reflux, burping, and gas. Prevents air bubbles from forming in milk or formula. Helps reduce fluid in the ear. Prevents nipple collapse. Replacement silicone nipple for Dr. Brown's Natural Flow baby bottles. All of our nipples are super-soft, made of high-grade silicone in Germany. exclusively for Dr. Brown's Natural Flow Standard baby bottles. Level-4 is ideal for feeding more aggressive eaters.