Propylene glycol is a terrific solvent, but it produces formaldehyde and carbonyl compounds when overheated. Both these are suspected carcinogens. Vegetable glycerin is also a great additive to vape juice as it produces a better taste and thick vapor. But it has the same issues as PG in that it produces cancerous compounds when heated. Many prefer VG because of the plume and flavor enhancement.
Medium Chain Triglycerides offer a fairly inert solvent for vape juice. There is a condition known as lipid pneumonia which can occur. This is where oil builds up in your lungs and affects your ability to breathe. This condition is temporary and treatable.
It can be argued that anything taken into your lungs is going to have potential risks involved. When compared with smoke a vape may be less toxic, but this is not known. Of the ingredients commonly used to manufacture vape juice, we choose the one with the least predictable risks. If it can produce a carcinogen in the presence of heat, we will avoid that product. MCT is used to make our products.