Medication that relieves pain but also slows respiration

The answer is; opioids

These painkillers relieve pain by competing with neurotransmitters in binding neurotransmitter receptors in the synapse. However, they don’t induce an impulse hence block the receptors from neurotransmitters. This working principle, however, is also problematic because it causes slow breathing because the drugs calm nerve activity even that which controls your breathing.