Common Word Confusions in OET Writing: Affect vs Effect, Prescribe vs Proscribe

“The medication did not affect any improvement in her symptoms.” Read quickly, that sentence sounds fine. Read carefully, it’s wrong — the writer meant “effect,” not “affect,” and the letter now says something subtly different from what was intended. Word confusions in OET writing like this one are some of the hardest Language errors to catch, because every word involved is spelled correctly. Nothing is underlined red. The sentence simply says the wrong thing, confidently, in a letter a doctor or nurse is relying on to be accurate.

Why do word confusions in OET writing survive so many proofreading passes?

Most Language-criterion errors that candidates catch themselves are the ones that look wrong — a missing article, an odd verb ending, a sentence that doesn’t quite parse. Word confusions don’t look wrong, because they aren’t misspellings. “Affect” and “effect” are both perfectly good English words. So are “prescribe” and “proscribe.” The error isn’t in the spelling; it’s in choosing the wrong one of two real words for the sentence you’re writing. Reading your own letter back rarely catches this, because your eye recognises a familiar, correctly-spelled word and moves on — it doesn’t automatically stop to check whether that word is the right one for this exact sentence.

This is also why the error is so persistent at every English proficiency level. Candidates who write fluent, complex clinical English make this mistake just as often as candidates working at a more basic level, because fluency doesn’t fix a fixed rule you haven’t learned — it just makes the sentence around the error read more confidently, which paradoxically makes the error easier to miss on a read-through.

What’s the difference between affect and effect?

Affect and effect are the single most common word confusion in OET letters, because the words are near-homophones in connected speech and differ by only one vowel in writing.

• Affect is a verb in the overwhelming majority of clinical sentences. It means “to influence” or “to have an impact on.”
  • ✅ “The infection has affected her mobility.”
  • ✅ “Poor sleep may affect his recovery.”
• Effect is a noun in the overwhelming majority of clinical sentences. It means “a result” or “an outcome.”
  • ✅ “The effect of the infection on her mobility was significant.”
  • ✅ “The side effects of the new medication should be monitored.”

A quick, reliable test: try putting “the” or “a/an” directly in front of the word. If that works grammatically, you need effect (“the effect,” “a positive effect”). If the sentence instead needs a subject doing something to an object, you need affect (“the infection affects her mobility”). This test covers almost every OET letter sentence, because the rarer reversed uses — “affect” as a clinical noun describing a patient’s displayed emotional state, and “effect” as a verb meaning “to bring about” (“the new protocol effected a change in discharge times”) — appear so infrequently in referral, discharge, and advice letters that they’re not worth the risk of defaulting to them.

What’s the difference between prescribe and proscribe?

Prescribe and proscribe are a lower-frequency error than affect/effect, but a more serious one when it happens, because the two words are closer to opposite in meaning than they are close in spelling.

• Prescribe means to authorise, recommend, or formally direct a treatment.
  • ✅ “The GP prescribed a course of antibiotics.”
  • ✅ “I would recommend this medication be prescribed at the lowest effective dose.”
• Proscribe means to forbid or prohibit something, typically for safety reasons.
  • ✅ “Aspirin is proscribed for patients with this bleeding disorder.”
  • ✅ “Strenuous activity was proscribed during the recovery period.”

Writing “proscribed” when you mean “prescribed” tells the reader that a drug or activity is forbidden when you actually mean it was authorised — and in a referral or discharge letter, that is exactly the kind of error that could lead a colleague to withhold treatment a patient is supposed to be receiving, or to permit something that was meant to be restricted. The clinical stakes of this particular confusion are higher than most Language errors, which is one reason it’s worth learning as a fixed pair rather than relying on how the word “feels” in the moment.

Affect/effect and prescribe/proscribe side by side

Does mixing these up actually cost Language marks?

Yes, under the conditions the OET rubric already distinguishes for every Language error: occasional versus systematic. One slip of affect for effect in an otherwise accurate 180-200 word letter may not move the needle — examiners are instructed to tolerate occasional errors that don’t block communication. The risk is that candidates who haven’t learned the fixed rule for a pair don’t make the error once; they default to the same wrong choice every time that grammatical slot comes up, which turns a single slip into a repeated pattern across the letter. That pattern is exactly what the Language criterion is designed to catch, and it’s graded independently of how strong your vocabulary or sentence structure is everywhere else.

The prescribe/proscribe pair carries an additional risk beyond Language marks alone: because the meanings are closer to opposite than similar, a single wrong word can misstate the clinical content of the letter, which is a Content concern on top of a Language one. A reader who receives “proscribed” when the writer meant “prescribed” isn’t just reading a grammatically awkward sentence — they’re reading the wrong instruction.

A self-check for word confusions before you submit

Because these errors pass both spellcheck and a normal read-through, they need a targeted check rather than a general proofread. Run this pass once your letter is drafted:

1. Search your letter for “affect” and “effect.” For each instance, ask: is something doing an action to something else (affect), or am I naming a result (effect)? Confirm the word matches.
2. Search for “prescribe” and “proscribe.” For each instance, ask: am I describing a treatment being authorised, or something being forbidden? These two meanings should never be interchangeable in context — if either reading is plausible, you’ve likely used the wrong word.
3. Read the sentence with the opposite word substituted in. If “the medication was proscribed” and “the medication was prescribed” would both make surface sense in context, stop and check the case notes again for which one is actually true.
4. Don’t trust how the word “feels.” Both pairs are near-homophones in speech, which means your internal sense of “that sounds right” is unreliable here specifically — this is a rule to apply mechanically, not a judgement to make by ear.
5. Check for repetition. If you’ve used affect or effect more than once in the letter, verify each instance independently — getting the first one right doesn’t guarantee the second one is also correct.

This takes under a minute once you know which two pairs to search for, and it catches an error type that a standard grammar check misses precisely because both words in each pair are valid English.

How this connects to the OET marking criteria

Affect/effect and prescribe/proscribe errors are marked under Language, one of the six official OET writing criteria alongside Purpose, Content, Conciseness & Clarity, Genre & Style, and Organisation & Layout. For the broader set of grammar patterns that most often cost Language marks — articles, tense consistency, subject-verb agreement, and more — see our OET writing grammar guide, and for the related skill of choosing precise, unambiguous wording more generally, see precision in OET writing. The official Language criterion page sets out how this criterion is scored band by band.

Word confusions are also a useful case study in why vocabulary range alone doesn’t protect a letter’s Language score — a candidate with a wide vocabulary who hasn’t learned these two specific fixed rules is just as exposed as a candidate with a narrower one, because the error isn’t about how much English you know, it’s about whether you’ve learned these particular pairs as rules rather than relying on instinct.

The honest limit of catching these yourself

Affect/effect and prescribe/proscribe are difficult to self-edit precisely because both words in each pair are correct, familiar English — your eye has no reason to pause on either one unless you already know to check. The free Grammar Checker is a reasonable first pass for surface-level errors, but catching a confidently wrong word choice that reads smoothly and passes spellcheck is exactly the kind of error that benefits from a second, trained reader. Our OET writing correction service has corrected 11,000+ letters since 2014 and reviews your actual letter against all six official criteria — including the word-level precision errors that are easiest to miss in your own writing because they never look wrong to the person who wrote them.