A month's supply of phentermine without a prescription ranges from $30 to more than $200, depending on the brand, form, and dosage. Generic pills are less expensive than brand-name preparations, and tablets are frequently less expensive than capsules. The average cash price for generic phentermine 37.5 mg tablets, for example, is $32.35, while the average cash price for generic phentermine 37.5 mg capsules is around $36. Qsymia is the most expensive phentermine pill on the market, with a monthly cash cost ranging from $194 to 240 depending on the dose.