Effective can be used as an adjective and also as a noun. Ineffective, on the other hand, can only be used as an adjective. They have opposite meanings too. Effective as an adjective means that it works. For example, “The campaign is effective.” This means that the campaign is a success. When people say, “The campaign is ineffective.” This means that the campaign has failed.
Effective can also be used for this sentence, “Purchasing in bulk is more cost-effective as compared to purchasing in small quantities.” Knowing the differences between the two will allow you to create sentences using the right terms properly.