Complex sentence: International organizations such as the WHO and those from the US have hesitated to recommend e-cigarettes for quitting smoking, because of limited evidence of effectiveness and safety, while some from the UK have recommended their use by smokers unwilling or unable to quit.
Split into simpler sentences: International organizations such as the WHO and those from the US have hesitated to recommend e-cigarettes for quitting smoking, because of limited evidence of effectiveness and safety,. Some from the UK have recommended their use by smokers unwilling or unable to quit.