Complex sentence: No e-cigarette has received FDA approval as a cessation tool but the evidence suggests that e-cigarettes can at least partially supply nicotine at concentrations that are enough to substitute for traditional cigarettes.
Split into simpler sentences: No e-cigarette has received FDA approval as a cessation tool. Though, the evidence suggests that e-cigarettes may at least partially deliver nicotine at concentrations that are enough to substitute for traditional cigarettes.