Why does the price for a prescription drug fluctuate?
Prescription drug prices, like those of most other retail products, fail to satisfy the law of one price routinely. The most obvious observation is that prices differ across pharmacies at a given point in time (http://www.nber.org/papers/w8548.pdf) but just like supermarkets give discounts, so do pharmacies. If you always buy from the same shop, you are bound to end up paying different amounts. 

The reasons for this are manifold. Different consumers have different abilities or willingness to store and buy large chunks when it is cheap and stochastic prices allows for price discrimination (see Sobel 84 for an early reference). The idea is simple. Myopic consumers arrive in each period and buy if $p&lt;V$, then leave the market. Otherwise, they die. Strategic consumers have a valuation of $v&lt;V$ but can wait if the price is above $v$. Firms compete in prices. In equilibrium, firms charge often $p=V$ to extract rents from myopic consumers but every now and then they offer $p&lt;v$ to attract all strategic consumers looking for prices.

Competition between stores may also result in random prices in a standard consumer search framework. Finally, retail prices may change because manufacturers prices are also random as a way to discipline pharmacies by encouraging consumers to shop.