Question

plastic bags helped the environment in some ways, but they also had many harmful effects on the environment.
antibiotics are another helpful invention, because they cure diseases by killing harmful bacteria.
, if antibiotics are used too much or incorrectly, they can lead to other
as a result
however

Explanation:

The previous sentence about antibiotics stated their helpfulness (curing diseases by killing harmful bacteria), and the current sentence introduces a contrasting idea (negative effects of overuse or incorrect use). "However" is used to show contrast, while "As a result" shows a cause - effect relationship which is not appropriate here.

Answer:

However