The function of the smooth endoplasmic reticulum is to perform a number of important metabolic functions within the cell. The exact function changes depending on the specific cell type. When not metabolizing a compound, the smooth endoplasmic reticulum acts as a storage site until metabolism begins again. Another important function is to detoxify compounds brought into the cell. A final but no less vital function is to package newly created proteins.

The general appearance of the smooth endoplasmic reticulum is a membrane folded over on itself, a feature that greatly increases the surface area where metabolic processes can occur without taking up excess space within the cell. The organelle gets its name from its appearance under a microscope; a similar organelle, the rough endoplasmic reticulum, has a collection of granules that produce proteins. One of the primary functions of the smooth endoplasmic reticulum is to produce lipids, an essential component of a cell's outer membrane. Another one of its functions common in all cells is the final production of glucose, an energy storage molecule.

The last common function among the smooth endoplasmic reticulum of all cell types is metabolizing natural and ingested toxins. Metabolizing naturally produced toxins is a continuous process; a cell would quickly die without this function. The smooth endoplasmic reticulum of liver cells metabolize a commonly ingested toxin: alcohol.

Other functions of the smooth endoplasmic reticulum depend on the cell type. In a muscle cell, for example, the smooth endoplasmic reticulum is responsible for releasing calcium ions when it receives a signal that the muscle cell must contract. The releasing of calcium ions causes muscle contraction. This function of the smooth endoplasmic reticulum is so specialized in muscle cells that it has a different name: the scarcoplasmic reticulum.

Another specialized function of the smooth endoplasmic reticulum is its role in the gonads of animal species. Instead of releasing calcium ions, the smooth endoplasmic reticulum in cells of the gonads assists in the last stages of steroid production. A unique feature of this process is that smooth endoplasmic reticulum and mitochondria, another organelle, work together to produce steroids. This fact is exceptional as most organelles work as independent units when maintaining the cell's life functions.

Certain genetic disorders interfere with the function of the smooth endoplasmic reticulum. In cystic fibrosis, the body cannot properly create a certain protein. This mutated protein becomes stuck in the smooth endoplasmic reticulum during packaging and is unable to leave the cell. As a result, the lungs become scarred and one's lifespan severely shortened.