For the past 30 years he has examined the clinical consequences of growth hormone deficiency and the effect of growth hormones. Jens Sandahl Christiansen and his colleagues were thus the first to introduce daily injections in the treatment of growth hormone deficiency, and also the first to complete a placebo-controlled study of the effect of growth hormones in adults.

He is also a Fellow at the Royal College of Physicians in Ireland and honorary member of the Polish Endocrine Society.