Injections can be of three kinds.

1.  Injections that are used as a substitute for food/water intake or to treat weakness. For example, glucose IV infusion. All scholars agree that this kind of injection breaks the fast.
2. Injections that are used as medicine for the treatment of a disease, missing which might worsen the condition. Taking such injections would not break the fast. According to scholars, such injections could be taken when needed because the medicine is pushed into veins or muscles and does not reach the stomach.
3. Injections that are used to draw out blood for testing purposes, do not affect the fast.

Reason:

Drawing out blood for test (blood work) does not affect the fast because cupping (hajamah) is also allowed while fasting.

Evidence:

Narrated by Ibn Abbas:

The Prophet  صَلَّى اللّٰهُ عَلَيْهِ وَسَلَّم was cupped while he was fasting.

Sahih Bukhari :1939

Note:

Cupping (hajamah) does not affect the fast as long as it does not cause weakness (due to excessive blood loss), in case of which the fast would need to be broken.

واللہ اعلم بالصواب