The IRS does not have a rule against experimental or unproven treatments (even though insurance companies may not cover them). What the IRS has is a qualified provider rule. If the treatment is prescribed by a qualified medical professional to treat a disease, the cost is deductible.
(However, procedures that are illegal are not deductible. For example, a doctor could not prescribe cocaine for example. As far as I know, stem cell therapies are investigational, and not FDA approved, but that is not the same thing as illegal.)
