My wife has breast cancer and takes chemotherapy that is injected into her vein once every two weeks. She also takes some drugs after each injection in order to alleviate the effects of the chemotherapy. Does a chemotherapy injection during the daytime in Ramadan invalidate fasting? Can she break her fast to take the medicine that she has to take after the injection? What is the expiation if she breaks her fast?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and messenger.

Taking intravenous injections in Ramadan does not affect the validity of fasting, as explained by the Islamic Fiqh Academy.

As for the medicine that your wife takes after the injection, she should delay it until night time, if possible. If she needs to take the medicine after the injection because otherwise her health condition will become worse or her recovery will be delayed, then she is allowed to take the medicine after the injection given that she has a valid excuse for breaking the fast.

In this case, she is required to make up for the day(s) that she comes to miss in Ramadan, and there is no expiation due on her. She will not be required to make up for the days if she is not able to, in which case she is required to feed a poor person for each day that she misses. 

Allaah Knows best.