Feedback inhibition occurs when the end product of a reaction interferes with the enzyme that helped produce it. The inhibitor does this by binding to a second active binding site that's different from the one attached to the initial reactant. The enzyme then changes its shape and can't catalyze the reaction anymore. This type of inhibition is done as a regulatory mechanism to meet the metabolic needs of the cell or organism.
Formaldehyde is a chemical compound that is widely used in industrial manufacturing and a number of other industries. Many people are familiar with it in the form of formalin, an aqueous solution of formaldehyde that is used as an embalming preservative. This chemical is toxic, known to cause cancer and a variety of other health problems, and for this reason, most people make an effort to avoid it.
Tretinoin is most commonly used as a form of acne treatment. It was the first retinoid developed for this type of topical use. Tretinoin is the best studied retinoid in the treatment of photoaging. It is used by some as a hair loss treatment and is a component of many commercial products that are advertised as being able to slow skin aging or remove wrinkles.