Contraceptives for women using hormones for women which are secreted within bodies of women and by the women have been gaining popularity over the past 40 years,when women got scared about using intrauterine devices due to plenty of adverse effects including blood loss adding on to the miseries of Anemic women.
Pharma industry has been using various combinations like only Progesterone homone or a reasonable dose of both estrogens + progesterones / adding Iron tablets in the packet to be taken during those 7 days when woman waits for a normal cycle.
How do pills act for 21 days?
First half of cycle produces feminizing hormone -Estrogen.
Second half of cycle produces both androgenic Progesterone + Estrogen.
Both hormones are essential to ripen the ova,to retain it for 48 hours and help it travel towards uterus for implantation and for fertilization.
A fertile woman will have cyclic changes from Puberty till Menopause.
If levels of hormones exceed the normal levels in produced by body , extra hormones work like poison for ova and prevent Contraception.
If the packet has only progesterone to be taken for 21 days , it can lead to
1) reduced periods
2) Breakthrough bleeding - spotting can start anytime without a reason or a warning and woman has to be prepared for those 7 days all through the month.
3) it can delay getting pregnant despite giving up pills for few months.
Injectables are progesterone hormones given Once in 3 months so that the forgetful woman is also covered from Not getting pregnant
During 1995 there were protests from women organizations as the fears were genuine that Our Women Were Subjected to erratic testing of hormones and side effects.
Fears were genuine.
Excess of hormones can cause Cancer also.
These injections continued at private places @ subsidized rates ranging from 200-300/- Once in 3 months and even Rural women could afford them and they took injections starting at age 25 + after completing their families of 2-3 kids.
This is Not Permanent Solution to number the families without giving a thought to the health of woman concerned.
For short gaps -yes for sure - maximum for one year or a bit more.
Injections /pills cannot be given to lactating mothers.
Plus point of using for short periods- it helps build precious blood but once family is complete do opt for harmless Newer Intra Uterine Contraceptive Devices(IUCD) / Ligations but Never Rely on Injections for life which can lead to Premature Menopause at 30+ and side effects of Premature Menopause.