As someone previously stated, quite correctly, it is safe to fire .380 ACP ammo in a pistol chambered for 9x18mm Makarov. He was also correct that doing the reverse, i.e. firing 9x18mm Makarov ammo in a weapon chambered for .380 ACP (also known as 9mm Kurz), would be extremely dangerous and should be attempted under no circumstances. As stated, you should not expect much in the way of accuracy when firing .380 ACP ammo, due to the difference in bullet diameters as detailed below.
The reasons for this are twofold: The .380 ACP is not loaded to pressures as high as some 9x18mm Makarov ammo, and therefore .380 ACP chambered firearms may not be able to withstand the higher pressures. Additionally, .380 ACP uses the U.S. "standard" .38 / .357 / 9mm bullet diameter of .355" for jacketed projectiles and .356-.357" for cast-lead bullets, whereas the 9x18mm Makarov uses .364-.367" diameter bullets. If you managed to get a 9x18mm Makarov round chambered in a .380 ACP firearm, the larger diameter bullet would become stuck in the chamber / throat / barrel and create massively increased pressure from even the normally hotter 9x18mm Makarov cartridge. This would, almost certainly, result in catastrophic failure of the weapon and very likely could injure or kill the firer and / or bystanders.