Sometimes, a regular sick person needs an injection or intravenous (IV) infusion on Shabbat. Since a subcutaneous injection does not necessarily cause bleeding, halakha views it no differently from other types of medicine: it is permitted for a sick person. If one is not sick but is in pain, he may ask a non-Jew to administer the injection.
However, a Jew may not administer an intravenous injection or infusion on Shabbat, because doing so causes bleeding, which some maintain is prohibited by Torah law. Thus, as long as the person is not dangerously ill, one should defer to this stringent opinion. If a port was installed beneath the skin (with a catheter surgically inserted into a vein) before Shabbat, one may attach an IV bag to it on Shabbat, even when there is no danger to life (see SSK 33:7). A non-Jew may administer an intravenous injection or infusion to a regular sick person.
If a wound or a needle needs to be disinfected with iodine or peroxide, one may not apply the disinfectant using cotton wool or a bandage, because squeezing liquid out of them is prohibited on account of Seĥita. Rather, one should pour the iodine or peroxide directly onto the surface that needs to be disinfected. Alternatively, one may apply it with a tongue depressor or synthetic, non-absorbent material.
If one knows before Shabbat that he will have to administer an injection on Shabbat, it is preferable that he prepare and disinfect the syringe and needle before Shabbat. If he forgot to prepare in advance, or if it was medically inadvisable to do so, he may prepare the shot on Shabbat, as this does not involve a Torah prohibition (see SSK 33:8-10).
A nursing woman whose breasts are engorged may express or pump her excess milk on Shabbat, on condition that she pumps it in such a way that the milk goes to waste, such as into the sink or into a container with soap in it so that the milk is immediately ruined. It is true that expressing milk on Shabbat in order to feed a baby is prohibited by Torah law as a tolada of Dash (SA 328:34; above, 10:17). However, when the milk goes to waste, the prohibition is only rabbinic, and to relieve pain the Sages permitted it (SA 330:8). One may use a manual pump for this purpose or an electric pump attached to a timer that was set before Shabbat. On Shabbat, when the timer activates the pump, the nursing woman may use it to pump her milk (SSK 36:22 n. 63). If the doctors believe that it is necessary for a baby to have breast milk, and the mother tries not to miss any opportunity to pump during the week, then she may also pump on Shabbat for the baby, as this is a case of saving a life (see SSK 36:22 and n. 67).