Typically, each Plan's formulary is organized into tiers, and each tier is associated with a set co-pay amount. Most formularies have between 3 and 5 tiers. The lower the tier, the lower the co-pay. For example, Tier 1 might include all of the Plan's preferred generic drugs, and each drug within this tier might have a co-pay of $5 to $10 per prescription. Tier 2 might include the Plan's preferred brand drugs with a co-pay of $40 to $50, while Tier 3 may be reserved for non-preferred brand drugs which are covered by the plan at a higher co-pay, perhaps $70 to $100. Tiers 4 and higher typically contain specialty drugs, which have the highest co-pays because they are generally more expensive. By 2011 in the United States a growing number of Medicare Part D health insurance plans had added the specialty tier.