Tetracyclines administered during the first years of life may produce serious unwanted side-effects, such as intracranial hypertension and, chiefly, abnormalities of the teeth and bones. The tetracyclines have a special affinity for certain metal cations, notably calcium, with which they form relatively stable complexes. When given to children or pregnant women they stain the deciduous and permanent teeth a lasting yellowish-brown or give rise to dental abnormalities; they may also slow down skeletal growth. For these reasons tetracyclines should not be prescribed to children and pregnant women unless absolutely necessary, and other classes of antibiotics should be preferred whenever possible.