Iron supplements can take between one week and one month before the benefits take effect. Results depend largely on the severity of the iron deficiency and the dosage levels, according to the Cleveland Clinic.
The rate of iron absorption is also affected by certain foods. Milk and calcium supplements are known to suppress iron absorption, and the Cleveland Clinic advises avoiding caffeine and antacids when taking iron. Vitamin C actually increases the uptake of iron, so the Clevelance Clinic recommends taking an iron supplement with a glass of citrus juice. If more than a month goes by without noticeable improvement, consult a health care professional to discuss the possibility of increasing the dosage.