Combination birth control pills contain a combination of two hormones, estrogen and progestin, while progestin-only pills contain (you guessed it!) progestin only. Progestin-only pills are also referred to as the mini-pill or POP.
Your doctor might recommend a progestin-only pill to you if you have certain health factors that make taking a combination pill risky.  For example, a progestin-only pill might be better for you if you are a smoker over 35 years of age, if you are breastfeeding, or if you have high blood pressure, migraines with aura, or a history of blood clots or stroke.
Important differences to take into account between progestin-only pills and combination pills:
Progestin-only pills must be taken at the same time every day to maintain pregnancy protection - their effectiveness decreases more drastically than that of combination-pills if taken irregularly. Put another way, while both types of pills are quite effective if taken regularly, combination pills are more forgiving if you miss or forget a pill.
There are no placebos in a progestin-only pill pack; just 28 progestin-only pills that are taken continuously. Most women still have a period while on the progestin-only pill, but many report that the pill makes it lighter.
Some women experience more mid-cycle spotting while on the progestin-only pill, but this often decreases after a few weeks or months of use.
Many experience fewer side effects from the progestin-only pill than from combination pills.