From WebMD:

Grapefruit is a citrus fruit. People use the fruit, oil from the peel, and extracts from the seed as medicine. Grapefruit seed extract is processed from grapefruit seeds and pulp obtained as a byproduct from grapefruit juice production. Vegetable glycerin is added to the final product to reduce acidity and bitterness.

Grapefruit juice is used for high cholesterol, “hardening of the arteries” (atherosclerosis), cancer, a skin disease called psoriasis, and for weight loss and obesity.

Grapefruit seed extract is taken by mouth for bacterial, viral, and fungal infections including yeast infections.

Grapefruit oil is applied to the skin for muscle fatigue, hair growth, toning the skin, and for acne and oily skin. It is also used for the common cold and flu (influenza).

Grapefruit seed extract is applied to the skin as a facial cleanser, first-aid treatment, remedy for mild skin irritations, and as a vaginal douche for vaginal yeast infections (candidiasis). It is also used as an ear or nasal rinse for preventing and treating infections; as a gargle for sore throats; and a dental rinse for preventing gingivitis and promoting healthy gums; and as a breath freshener.

Some people inhale grapefruit vapors to help the body retain water, for headache, stress, and depression. Grapefruit seed extract vapor has also been inhaled for the treatment of lung infections.

In food and beverages, grapefruit is consumed as a fruit, juice, and is used as a flavoring component.

In manufacturing, grapefruit oil and seed extract are used as a fragrance component in soaps and cosmetics; and as a household cleaner for fruits, vegetables, meats, kitchen surfaces, dishes, etc.

In agriculture, grapefruit seed extract is used to kill bacteria and fungus, fight mold growth, kill parasites in animal feeds, preserve food and disinfect water.

It’s important to remember that drug interactions with grapefruit juice are well documented. The chemistry of the grapefruit varies by the species, the growing conditions, and the process used to extract the juice. Before adding grapefruit to your diet or your list of natural medicines, check with your healthcare provider if you take medications.

How does it work?

Grapefruit is a source of vitamin C, fiber, potassium, pectin, and other nutrients. Some components might have antioxidant effects that might help protect cells from damage or reduce cholesterol.

It is not clear how the oil might work for medicinal uses.