Effective and effectual are close synonyms, both meaning something works or produces the intended result. The main practical difference is that effective has a second, common sense: officially in force from a stated date, as in a policy that is effective from next Monday. Effectual does not carry that second sense.
The core difference
Both words mean producing the intended result, but only effective also means officially in force as of a given date.
- effective — producing the intended result, or officially in force from a stated time: The new rules are effective from next Monday.
- effectual — producing the intended result; entirely adequate to the purpose: Her effectual handling of the crisis restored calm within hours.
How to tell them apart
In everyday writing, effective is by far the more common choice and does the job in almost every case, including describing a medication, a method, or a person's approach. Effectual is rarer and tends to appear in more formal or slightly old-fashioned writing, but it means essentially the same thing when describing whether something achieves its purpose.
The one place they clearly diverge is dates and rules: only effective is used to say when something officially starts, as in effective immediately or effective from 1 January. If you are describing a start date rather than how well something works, effective is the only correct choice.
