Compounding pharmacists throughout the world are compounding a special type of capsule called "slow-release." This type of capsule is a compounding pharmacy application of the commercial hydrophilic matrix tablets. It is a relatively simple system that allows formulating a robust, reliable, and consistent drug system based on 30% w/w to 40% w/w of specific types of hydroxypropyl methylcellulose. The main purpose of these capsules is to attenuate the drug release when there is a clinical need and no commercial medication exists. Five in vitro trials verified and proved this kind of preparation can be compounded by specialized pharmacists achieving substantial attenuation of drug release that resembles the pharmacokinetic profiles of commercial slow-release medications. An in-depth explanation of the mechanism of action of the slow-release capsules is provided. Since the pharmaceutical calculations needed to compound this preparation are time consuming, a suggestion of a faster way to perform these calculations by using a special Excel spreadsheet is provided. The article demonstrates a special table with a comparison between the specifications, results, and conclusions of the five in vitro trials that evaluated the pharmacokinetic rates of compounded slow-release capsules. The regulatory aspect of compounding slow-release capsules is also discussed.