Cyproheptadine HCl is primarily used as an antihistamine to treat allergic reactions, such as hay fever, skin rashes, and itching. It works by blocking histamine receptors, reducing symptoms like sneezing, runny nose, and watery eyes. Additionally, it is prescribed to manage symptoms of serotonin syndrome, a condition caused by excessive serotonin activity, due to its serotonin antagonist properties.

In some cases, it is used as an appetite stimulant, particularly in individuals experiencing weight loss or poor appetite due to medical conditions like cancer or HIV. Its ability to increase appetite makes it beneficial for patients needing to gain weight or improve nutritional intake.

Cyproheptadine HCl is also utilized in the treatment of migraines and cluster headaches, as it helps alleviate symptoms by affecting blood vessel dilation and serotonin levels. In veterinary medicine, it is sometimes administered to animals for similar purposes, such as managing allergies or stimulating appetite.

It is important to note that its use should be guided by a healthcare professional, as it may cause drowsiness or other side effects.