What does Combining drugs leads to?

Combining drugs leads to drug synergy, where the combined effect is greater than the sum of the individual effects.

Since we're in the illicit-drugs realm rather than the licit one (where a doctor might prescribe two separate antibiotics to exploit synergism to kill a really nasty infection), one famous synergism is mixing booze and barbiturates, which used to be a popular way for women to attempt suicide.