Natural supplement retailer GNC (GNC) is falling after William Blair analyst Mark Miller downgraded the stock to Market Perform from Outperform in a note to investors earlier today. GNC's strong sales in the first half of 2012 increase the risk that the company's sales growth will decline in upcoming quarters, Miller believes. Meanwhile, Amazon.com (AMZN) is selling more of GNC's products at lower prices, creating additional competition for GNC. Since GNC doesn't sell its products to Amazon, the online retailer is likely receiving the supplements from GNC franchisees, Miller explains. Meanwhile, GNC is seeing increased competition from other brick and mortar retailers, including Vitamin Shoppe (VSI) and Walgreen (WAG), reports the analyst. In early trading, GNC dropped $1.95, or 5.43%, to $33.99.