Leech Cards: What They Are and How to Kill Them

A leech is a flashcard you keep failing — it eats review time and gives nothing back. What causes leeches, and the four fixes that actually work.

A leech card is a flashcard you fail over and over: it keeps coming back, you keep missing it, and it slowly drains your review sessions — hence the name. The term comes from the Anki community, where the app formally tags a card as a leech after enough repeated lapses, but the phenomenon exists in every flashcard system, paper included. If you study with spaced repetition for more than a few weeks, you will breed some.

Why leeches matter

The math is ugly: failed cards return on short intervals, so a handful of leeches can consume a quarter of your daily reviews while contributing almost nothing to what you actually retain. Worse, they demoralize — the daily whiff of the same five impossible cards is a top reason people quit their decks entirely. Killing leeches is some of the highest-leverage deck maintenance there is.

What actually causes them

• The card is badly written. The most common cause by far: it asks multiple things at once, depends on context you don't have in front of you, or is a recognition-style prompt that never built real recall. The design rules in how to make good flashcards prevent most leeches before they hatch.
• Interference. Two similar cards fight each other — "affect vs effect", two look-alike drug names, two dates in the same decade. Each one blocks the other's retrieval.
• You memorized without understanding. An orphan fact with no structure around it has nothing to stick to — see where flashcards fail.
• The material is genuinely arbitrary. Some things (random ID codes, irregular verbs) carry no logic and simply need stronger encoding.

Four fixes, in the order to try them

1. Rewrite the card. Split multi-fact cards into singles; flip recognition into production; add the minimal context that makes the question answerable. This alone kills half of all leeches.
2. Attack interference head-on. Make a contrast card: "affect vs effect — which is the verb?" Studying the confusable pair together, as a discrimination, is the standard cure — the same logic behind interleaving.
3. Add a memory hook. A vivid mnemonic, an absurd image, a mini memory-palace stop — arbitrary material needs artificial structure, and thirty seconds of encoding beats thirty failed reviews.
4. Delete it. Seriously. If a card has failed ten times and the fact isn't load-bearing for your exam or job, the honest cost-benefit says let it go. A deck is a tool, not a museum of everything you once typed.

Preventing the next generation

Leeches are feedback: they're telling you which cards were badly made or which concepts you skipped understanding. Notice the pattern in your own leeches — same subject? same card style? — and adjust how you write cards there. A few minutes of Feynman-style explanation before carding a hard concept inoculates it.

Frequently asked questions

How many failures make a card a leech?

Anki's default tags a card after 8 lapses. The number is convention, not science — if a card has annoyed you all week, it's a leech regardless of count.

Should leeches be suspended or deleted?

Suspend (set aside) when the fact matters and you plan to re-encode it properly; delete when it doesn't. The worst option is the default: letting it keep leeching.

Are leeches a sign spaced repetition isn't working?

The opposite — the system is correctly isolating exactly which items your memory rejects. That's information; use it to fix the card, not to abandon the method (how the scheduling works).

Do leeches exist with paper flashcards?

Absolutely — they're the dog-eared cards permanently stuck in your "again" pile. Paper just doesn't count the failures for you.