PRESCRIBING

and prescribing assessment tools

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path: PRESCRIBING

Why is prescribing so important?

Prescribing is an integral part of a General Practitioner’s work and several high profile cases have been published when qualified doctors have made catastrophic errors.   Because there are a large number of patient deaths relating to medication errors.   The only way this will improve is if we start regularly reviewing and analysing our prescribing habits.  The best way of looking at an identified need is to pause and reflect on what we are doing rather than bury our heads and just continuing as we are.  Would you agree?

We know GP trainees appear to assessed left-right-and centre.  But this is another important area where we can better ourselves.  Qualified GPs and their practices are often asked to look at their prescribing habits in some way or another and we feel therefore trainees should too.

The official MRCGP Prescribing Assessment?

Yes.    All trainees who start ST3 in August 2019 will need to complete a prescribing assessment by 31 January 2020.   It can replace 2 Case Based Discussions in the ST3 year – so the total CBDs required in ST3 will be 10 not 12!  All other trainees are, of course, welcome to use the assessment as appropriate to their learning needs.

There is no set standard as it is designed as a learning exercise; however if no errors are highlighted and if no learning is identified this would raise concerns, as to date this has never been the case.

MRCGP Prescribing Assessment - how will it work?

For the official MRCGP prescribing tools  and supporting documents to help you, see the resource items under the QUICKLINKS section at the top of the page.  They are reproduced below for your convenience.

Prescribing Pearls

If  you have  a prescribing pearl to offer, please submit it via the comment box below and I will add it to the list.

  • Try not to give “a pill for every ill” – promote self-help and less reliance on the doctor for small illnesses.
  • Drugs which often interact with other drugs and to be cautious off when adding more medication…
      • Gentamycin
      • Warfarin
      • Lithium
      • Digoxin
      • Theophylline
      • Methotrexate
      • Phenytoin
      • Insulin
      • Cyclosporin
  • Bone pain from bone mets – usually responds better to NSAIDs than opioids.
  • Know the features of opioid toxicity (confusions, picking at things in mid air, hallucinating).   Most likely if opioid doses increased too much too suddenly.  
  • If a patient needs more pain relief, can generally go up by 1/3rd total 24h dose. Check with BNF. 
  • If you have increased the regular morphine dose, remember to increase the breakthrough dose too.  
  • Do not prescribe Verapamil with Diltiazem (mnemonic VD).
  • Reduce diuretics in the elderly during Summer time (otherwise can dehydrate).
  • Know the features of Digoxin toxicity (and monitor blood levels).
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  • Thyroxine replacement – be guided by the TSH level generally when deciding whether to increase, decrease or maintain the levothyroxine dose.
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  • Remember, Lansoprazole cover for those who will be on long term NSAIDs or >= age 45
  • Antidepressants take about 2-4  weeks to work.  So tell patients to persevere for 4 weeks when starting or changing dose regimes.
  • Do not over emphasise that some antidepressants can make you feel worse than better initially.   If you over-egg this, then more patients are likely to experience this because of the nocebo effect (opposite of the placebo effect).
  • If coming off SSRIs – do it slowly.  Otherwise withdrawal effects.
  • No Paroxetine/Fluoxetine for those on Tamoxifen – use Citalopram instead. Paroxetine and fluoxetine should not be prescribed for depression or hot flashes in women who have had breast cancer and are now taking tamoxifen to prevent a recurrence. Citalopram or venlafaxine should be considered instead.  This message comes from a study showing that paroxetine, by interfering with the metabolism of tamoxifen, reduces or abolishes its protective effect against breast cancer recurrence, and that women taking both drugs have an increased risk for death from breast cancer. Paroxetine is a strong inhibitor of the CYP2D6 enzyme that converts tamoxifen to its active metabolite, reducing the amount of active drug that is released
  • Do not prescribe B-Blockers in Asthma
  • Know the features of Lithium toxicity & monitor blood levels.
  • Naproxen vs Ibuprofen vs Diclofenac
  • Don’t use diclofenac (high risk of GI ulcers and bleeds).  Naproxen safer, even more so than Ibuprofen.   May need to prescribe with PPI esp if age =>45 (see CKS guidance).
  • Do not prescribe pentins in general (e.g. gabapentin, pregabalin)
  • Do not generally prescribe diazepam for back pain. 
  • Capsaicin – remember, if using, only use a small amount to rub in.   Too large a blob, and all that will result is chilli-like burning.
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  • Do not prescribe B-Blockers in Asthma
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Cytochrome P450 made easy

What is it?

When we mention the cytochrome P450 system, most doctors just want to turn around and run away! So, let’s demystify it once and for all.    The cytochome P450 (CYP45) system is basically the liver’s enzyme system.  The CYP450 is so called because it is found within the membranes of a cell (hence CYTO).  It contains a haem pigment (hence CHROME and P).  And it absorbs light at a wavelength of 450nm.    It has quite a number if functions…

  • It is essential for the production of lots of things in the body including our cholesterol and natural steroids. 
  • It also helps detoxify the body of foreign chemicals
  • It helps with the metabolisation of drugs (so they don’t linger about too long)

Why is it important?

Because so many drugs (including nutrients and herbal therapies) are metabolised via the CYP450 system.  This system can be inhibited or induced by drugs.   And that then leads to drug-drug interactions and all sorts of adverse reactions.   Drugs that cause CYP450 drug interactions are referred to as either inhibitors or inducers.

  • Inducers can increase the rate of another drug’s metabolism by x2/x3 fold over the period of a week!   So, when an inducing agent is prescribed with another drug, the dosage of that other drug may need to be increased.  Otherwise, the HIGHER rate of metabolism means the medication doesn’t last long and its therapeutic effect is reduced leading to therapeutic failure of that medication.
  • Inhibitors can decrease the rate of another drug’s metabolism by x2/x3 fold over the period of a week!   So, when an inhibitor agent is prescribed with another drug, the dosage of that other drug may need to be reduced.  Otherwise, the LOWER rate of metabolism means the medication last longer and its therapeutic effect is enhanced leading to harmful adverse effects.  

And what makes it worse, is that there is genetic variability in about 7% of people.  For example, many clinicians believe that post menopausal women recieving tamoxifen for early breast cancer should be tested for their type of CYP2D6 genotype as it may be valuable in selecting the type of adjuvant homonal therapy to offer.  And of course CYP2D6 inhibitors should be avoided in tamoxifen-treated women.  

And patients come to us with often co-morbidity.  It would be nice if the world was just full of people with just the one thing.  But it isn’t.  It’s a lot more complex.  It’s not uncommon for a patient withy hypertension and bad lipids to present to us at some point with depression, for instance.  Did you know that several antidepressants are CYP450 inhibitors (i.e. reduce metabolism of other drugs).  The effect on drugs like halperidol or metoprolol will then be accentuated.  

In the case of grapefruit juice, there are numerous medications known to interact with grapefruit juice including statins, antiarrhythmic agents, immunosuppressive agents, and calcium channel blockers. Furthermore, the inhibition of the enzyme system seems to be dose dependent; thus, the more a patient drinks, the more the inhibition that occurs. Additionally, the effects can last for several days if grapefruit juice is consumed on a regular basis. Luckily, the effect of this is not seen with other citrus juices.

Examples of INDUCERS 

(i.e. speeds up metabolism of other drug, so it may not be as effective)

Mnemonic: SCRAP GPS

  • Sulphonylureas
  • Carbamezapine
  • Rifampin
  • Alcohol
  • Phenytoin
  • Griseofulvin
  • Phenobarbital
  • St Johns Wort

Examples of INHIBITORS 

(i.e. slows down the metabolism of other drug, so it’s effects may be accentuated)

Mnemonic SICKFACES.COM G

  • Sodium valproate
  • Isoniazid
  • Cimetidine
  • Ketoconazole
  • Fluconazole
  • Alcohol..binge drinking
  • Chloramphenicol
  • Erythromycin
  • Sulfonamides
  • Ciprofloxacin
  • Omeprazole
  • Metronidazole
  • Grapefruit Juice
  • Warfarin
  • COCP
  • Corticosteroids
  • Tricyclics
  • Statins
  • Theophylline
  • Pethidine

Please leave a comment if you have a tip, spot an error, spot something missing or have a suggestion for a web resource.
And of course, if you have developed a resource of your own, please email it to me to share with others.

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