If it’s classed as birth control pills, then it’s still going to protect u from pregnancy as long as ur taking them properly, regardless of what the dose is. They legally can’t be classed as & called birth control pills if they’re not strong enough to actually prevent pregnancy.

If you’re on a low dose for the purpose of helping with periods or any other specific medical reason, no. I wouldn’t trust using just that unless it was a normal dose and prescribed to prevent pregnancy.