Praise be to Allah.
Islam encourages us to have many children, and the Prophet of Allah Shu’ayb (peace be upon him) reminded (his people) of this blessing and said to them: “And remember when you were but few, and He multiplied you” [al-A’raaf 7:86]. And it was narrated from Ma’qil ibn Yasaar (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: “Marry those who are loving and fertile, for I will be proud of your great numbers before the nations.” Narrated by Abu Dawood, 2050; classed as saheeh by al-Albaani. See also question no. 13492.
Being sterilized permanently may mean one of two things:
1 – It may be done out of necessity, such as if it is determined by trustworthy medicinal evidence that pregnancy will pose a danger to the mother’s life, and there is no hope of a cure, so permanent sterilization will ward off that danger. In this case it is permissible to be sterilized.
2 – When there is no need for it. Undoubtedly in this case it is a criminal act and a major sin, because it is a transgression against the creation of Allah for no reason, and preventing the production of offspring which was encouraged by the Prophet (peace and blessings of Allah be upon him), and being ungrateful for the blessing of children which Allah bestows upon His creation.
It says in al-Insaaf: It says in al-Faa’iq: And it is not permissible to do anything that will put a stop to pregnancy. (1/383).
The Islamic Fiqh Council stated the following in its report no. 39 (1/5):
It is haraam to sterilize both men and women, if there is no necessary reason for doing so unless there is a necessity which is to be determined according to the guidelines set out by sharee’ah. It is permissible to take temporary measures to space pregnancies or prevent them for a limited period of time, if there is a legitimate shar’i need to do so, so long as this decision is made on the basis of mutual consultation and approval between the spouses. That is subject to the condition that no harm should result from that, and that the means should be acceptable according to sharee’ah, and that there should be no harm caused to an existing pregnancy. End quote.
Based on this, if your preventing pregnancy is for a legitimate reason as outlined above, then you do not have to do anything. But if it was not for a necessary reason, then you have done something which is haraam, and you have to repent sincerely to Allah and stop taking these injections immediately. If there is anything that will reverse their effect without causing harm, then you must take it.
And Allah knows best.