Prescription

Clinical Pharmacology

Why use drugs?

To improve quality or quantity of life

To cure, suppress or prevent disease

Before starting treatment !

Decide whether a drug is necessary. If it is:

1. What are you hoping to achieve?

2. Will the drug chosen will bring this about?

3. What other effects the drug might have – could these be harmful?

4. Does benefit outweigh risk?

Risk versus benefit

Negligible risk

Acceptable risk

Unacceptable risk

How should you choose a drug?

Safety & tolerability Efficacy Cost-effectiveness

Why take a drug history?

can cause disease (early or late) can conceal disease can give diagnostic clues can interfere with diagnostic tests history can assist treatment choice

Drugs:

History of adverse reactions?

“I can’t take antibiotics, they make me ill, doctor”

Which specific drugs? When? Actual adverse reaction, beware “allergy” Similar drugs since?

Reporting of adverse drug reactions

Yellow card system All suspected reactions to new drugs Serious reactions to established drugs Committee on Safety of Medicines (CSM) Medicines and Healthcare Devices Regulatory

Authority (MHRA)

Responsibilities of the physician?

Not to be ignorant of existing knowledge or important new developments

To adopt new developments of proven value To prescribe accurately and clearly To avoid inappropriate prescribing To tell patients what they need to know To accept responsibility for one’s actions

What should you tell the patient (1)?

About the condition and why we are treating it The name of the medicine

– It may help to write this down for the patient The objective of the treatment Whether and how the patient will judge benefit How soon benefit can be expected

What should we tell the patient (2) ?

How and when to take the medicine What to do about a missed dose How long the medicine is likely to be needed How to recognise ADRs and how to respond to

them Important interactions with e.g. alcohol and other

medicines

The prescription – pitfalls

Doses Route

– Choose an appropriate route e.g. is the patient vomiting?

– Care with doses with different routes e.g. Penicillin 1.2gg iv versus 1.2mg intrathecal

– Do not use the im route if patient is anticoagulated

The prescription – pitfalls

DosesVancomycin

–Cl difficile 125mg qds PO

–Staph aureus 1g bd IV

The prescription – pitfalls

Doses Dose reduction

– Elderly, renal failure, hepatic failure Children

– Dose often calculated by weight

– Paediatric pharmacopoeia available

The prescription – pitfalls

Rate Bolus vs Infusion

– Vancomycin “red man syndrome”

– Frusemide and ototoxicity Minutes or hours ml or mg

– GTN 50mg in 50ml (5% dextrose) at 1 to 10 ml per hour

The prescription – pitfalls

Cost Cl Difficile

– Metronidazole £1-50

– Vancomycin £105-00

Contra-indications

Absolute– Beta blockers and asthma

– Misoprostol and pregnancy

Relative– Ciprofloxacin and epilepsy

Interactions

Two drugs together – Beta blockers (IV or PO) and verapamil (IV)

– Phenytoin and the OCP

– Ciprofloxacin and theophylline

– Enzyme inducers vs. enzyme inhibitors Nutrition

– NG feeding and phenytoin

Diseases– Ampicillin and EBV

Special situations Pregnancy

– Avoid all drugs if possible – but especially ACEI, gentamicin, carbimazole, isotretinoin, misoprostol

Breast feeding– Avoid most drugs – especially

ciprofloxacin, amiodarone

Renal / Hepatic impairment– Avoidance, or change in dose –

gentamicin, opiates

How can we contain cost?

Appropriate prescribing Generic prescribing Therapeutic substitution Timely discontinuation

However, many patients do not receive treatment from which they would clearly benefit (e.g. in hyperlipidaemia and heart failure)

Compliance

Also: adherence / concordance / co-operation

25-50% of patients take < 90% of prescribed dose

May be due to poor understanding, so cannot comply

Can occur in the face of good understanding

Main reasons for poor compliance

Poor doctor-patient relationship Lack of motivation Forgetfulness Deliberate intention Lack of information Frequency & complexity of drug

regimen (and total number of drugs) Adverse drug reactions

How can we improve compliance?

Form a ‘partnership’ with the patient Provide oral and written information Rationalise drug therapy Plan treatment around the patient’s life Use ‘patient-friendly’ packaging Use combined fixed-dose & SR formulations See the patient regularly Use dosette box if appropriate

Summary

Prescribing is an important responsibility Potential to do harm as well as good Good prescribing is fundamental to

being a good doctor

“Poisons in small doses are the best medicines; and useful medicines in

too large doses are poisonous”

William Withering 1789

Drug Calculations and Prescriptions

Question 1 An asthmatic presents with a severe exacerbation of

asthma. She has had a dose of steroid, high flow oxygen and has had a few nebules of Salbutamol and Atrovent. However, her peak flow is still very low and she remains tachypnoeic. You are the admitting doctor and after review by your senior, you are asked to prescribe intravenous Aminophylline.

A) what important feature in the history do you have to elicit before this?

B) her weight is 60kg – BNF dose is 5mg/kg loading given over 20 minutes and 500 microg/kg/hour maintenance dose in saline or 5% dextrose

Prescribe this on the infusion chart. Write out a prescription for the nurses to begin this emergency drug.

Answer

a) Check not on oral Theophylline. If so do not give a loading dose and check plasma theophylline levels.

  b) Loading 300mg bolus over at least 20 minutes. Written on yellow

infusion chart as:

Date: 21/10/5 Line: IV Type of fluid: 5% dextrose or 0.9% Saline

Additives: Aminophylline 300mg Volume: 100 ml Rate: over 20 mins.

SIGN!!

Maintenance = 30mg/hour. Written on yellow infusion chart as:

Date: 21/10/5 Line: IV Type of fluid: 5% dextrose or 0.9% saline

Additives: Aminophylline 500mg Volume: 500ml Rate: 30ml/hour

SIGN!!

or 500mg in 250 ml dextrose/saline at a rate of 15ml/hour.

ADDENBROOKE'S NHS TRUST SURNAME Bloggs Hospital No 12345

PARENTERAL INFUSION PRESCRIPTION AND RECORD OF ADMINISTRATIONFIRST NAMESFred Date of Birth 9.6.54

CONSULTANTKV WARD C7

PRESCRIPTION ADMINISTRATION RECORD

DATE LINE TYPE OF FLUID/BLOOD ADDITIVES Volume RATEOTHER

INSTRUCTIONS/DRUGSDR's

SIGNATUREBATCH No

DATE & TIME STARTED

NURSES INITS

21.10.2005 IV Sodium Chloride 0.9% Aminophylline 300mg 100ml over 20mins Loading dose A Doctor 12345 4.1.05 6am A Nurse

21.10.2005 IV Sodium Chloride 0.9% Aminophylline 500mg 500ml 30ml/hour Maintenance dose A Doctor 31425 4.1.05 2pm A Nurse

DATE & TIME DISCONT

4.1.05 2pm

4.1.05 10pm

Question 2

A young man has fallen down and sustained a laceration to his head. He presents to A&E and has a wound that will require suturing under local anaesthetic. The Sister hands you a box of vials of Lidocaine 2%. The patient weighs 70kg. Work out the maximum volume of lidocaine 2% you can use as a local anaesthetic in this patient. – BNF recommends a maximum dose of 200mg in any patient

What is the maximum dose in mls? Write out a prescription for this on the appropriate chart

Answer

2% lidocaine = 2g in 100 ml

= 2000mg in 100ml

= 20 mg in 1 ml

Max dose is 200mg (in solutions with Adrenaline – max dose is 500mg) hence maximum volume is 10ml.

Write out on once only prescription chart as:

Date: 21/10/2005 Drug: Lidocaine 2% Dose: 200mg

Route: S/C Time: as and when given

and SIGN!!

Answer – Once Only side of Prescription Card

Route/ Time toother directions be given Initials Time

21.10.05 Lidocaine 200mg S/C 1350 a.Doctor AD 1350

Subsequently Given/

DRUGS NOT ADMINISTERED

Date Time Drug Initials Reason Codes

Given by

CHECK FOR ALLERGY STATUS ON PAGE 1

Once Only Prescriptions

Pharm. Date Drug (approved name) Dose Signature

Question 3

An elderly man with known epilepsy presents in status epilepticus. He has already had rectal and intravenous Diazepam but these have failed to settle his convulsions. After review by the on call SpR, a decision is made to write him up for intravenous Phenytoin – loading then maintenance dose. The BNF states: For IV infusion (use saline 0.9%) in status epilepticus 15mg/kg at a rate not exceeding 50mg/minute as a loading dose; maintenance doses of about 100mg thereafter at intervals of 6 – 8 hours. Work out the correct infusion rates for the loading and maintenance doses.

Write up an infusion of Phenytoin on the infusion chart. The patient weighs 80kg. Also write up the regular maintenance dose on the appropriate drug card.

Answer

Loading = 1200mg. (80kg x 15mg/kg). Admin rate not more than 50mg/min hence write as: eg: 1200 mg Phenytoin in 200 ml saline 0.9% (= 6mg/ml) at a rate of 8ml/minDate: 21/10/05 Line: IV Type of fluid: Saline 0.9% Additives: Phenytoin 1200mg Volume: 200ml Rate: 8ml/min SIGN!!

or 1000 mg in 100ml saline (=10mg/ml) at 5ml/min followed by200mg in 20 ml saline “ “ at 5ml/min.= total 1200mg

Maintenance = 100mg tds or qds IV in 100 ml n/saline Drug: Phenytoin Dose: 100mg Route: IV Start Date: 21/10/2005 Circle frequencies eg 8,14,22

Additional Instructions: in 100 ml saline SIGN!!!

Parenteral Infusion ChartADDENBROOKE'S NHS TRUST SURNAME Bloggs Hospital No 12345

PARENTERAL INFUSION PRESCRIPTION AND RECORD OF ADMINISTRATIONFIRST NAMESFred Date of Birth 9.6.54 Sex

CONSULTANTKV WARD C7

PRESCRIPTION ADMINISTRATION RECORD

DATE LINE TYPE OF FLUID/BLOOD ADDITIVES Volume RATEOTHER

INSTRUCTIONS/DRUGSDR's

SIGNATUREBATCH No

DATE & TIME STARTED

NURSES INITS

21.10.05 IV Sodium Chloride 0.9% Phenytoin 1200mg 200ml 8ml/min Loading dose A Doctor

OR ALTERNATIVELY

21.10.05 IV Sodium Chloride 0.9% Phenytoin 1000mg 100ml 5ml/min Loading dose A Doctor

21.10.05IV Sodium Chloride 0.9% Phenytoin 200mg 20ml 5ml/min Loading dose A Doctor

DATE & TIME DISCONT

Maintenance dosing – Prescription Chart

Surname Hospital No

Bloggs 162534

First Names Date of Birth Sex

Fred 3.6.54 M

Consultant Ward

Mr K Varty C7

Weight

Height

DRUG SENSITIVITIES

Doctor must also enter this information on FRONT of case folder Drugs must not be administered unless this box has been completed

Date Drug/Substance

21.10.05 None known

Prescription Chart

DRUG (APPROVED NAME)6

8 ANDose Route Start Date Stop Date

12

100mg IV 20 10 0514

Signature18

22

Regular Prescriptions Month and date

Tick times or enter other times 26th

Pharm

Additional Instructions

Phenytoin

A Doctor

Question 4

A young girl (weight 50kg) has taken 30 tablets of Paracetamol 500mg. She is brought into casualty 8 hours after the overdose. She admits to taking the overdose with alcohol. Her paracetamol levels indicate that she is at high risk of hepatocellular necrosis so the Regional Poisons Unit advises you to commence an infusion regime of N-Acetylcysteine (Parvolex). The BNF states for IV infusion in 5% glucose, initially 150mg/kg in 200 ml over 15 mins, followed by 50mg/kg in 500ml over 4 hours then 100mg/kg in 1000ml over 16 hours.

Answer

N-Acetyl 7500mg in 200ml 5%glu over 15 mins then 2500mg in 500ml over 4 hours then 5000mg in 1000ml over 16 hours

Write out on yellow infusion card as:

Date: 7/11/3 Line: IV

Type of fluid Additives Vol Rate5% dextrose N-Acetlycysteine 7500mg 200ml over 15 minutes5% dextrose N-Acetylcysteine 2500mg 500ml over 4 hours5% dextrose N-Acetylcysteine 5000mg 1 litre over 16 hours

And SIGN!!

Parenteral Infusion Card

ADDENBROOKE'S NHS TRUST SURNAME BloggsPARENTERAL INFUSION PRESCRIPTION AND RECORD OF ADMINISTRATION

FIRST NAMESFred

CONSULTANTKV

PRESCRIPTION ADMINISTRATION RECORD

DATE LINE TYPE OF FLUID/BLOOD ADDITIVES Volume RATEOTHER

INSTRUCTIONS/DRUGSDR's

SIGNATUREBATCH No

DATE & TIME STARTED

21.10.05 IV 5% DEXTROSE N-ACETYLCYSTEINE 7500MG 200ml over 15 mins A Doctor

21.10.05 IV 5% DEXTROSE N-ACETYLCYSTEINE 2500MG 500ml over 4 hrs A Doctor

21.10.05 IV 5% DEXTROSE N-ACETYLCYSTEINE 5000MG I litre over 16 hrs A Doctor

Question 5

An elderly lady presents with confusion, fits and altered behaviour associated with a low grade pyrexia. Further investigations go on to reveal she has herpes encephalitis. The decision is made to start intravenous Acyclovir. Work out the dose for this 65kg woman and write out a prescription on the drug card.

The BNF suggests 10mg/kg every 8 hours for simplex encephalitis

Answer

650mg Aciclovir in 150 or 200 ml saline/glucose (ie 5mg/ml or less) tds over 1 hour for total 10 days

On regular drug card:

Drug: Aciclovir

Dose: 650mg

Route: IV

Start Date: 21/10/05

Additional instr: in 200 ml saline 0.9%

Freq: Circle 8,14,22

SIGN!!

Maintenance - Prescription Chart

Surname Hospital No

Bloggs 162534

First Names Date of Birth Sex

Fred 3.6.54 M

Consultant Ward

Mr K Varty C7

Weight

Height

DRUG SENSITIVITIES

Doctor must also enter this information on FRONT of case folder Drugs must not be administered unless this box has been completed

Date Drug/Substance

21.10.05 None known

Prescription Chart

DRUG (APPROVED NAME)6

8 ANDose Route Start Date Stop Date

12

650mg IV 21 10 05 14Signature

18

Aciclovir

A Doctor Pharm

Tick times or enter other times 26th

Regular Prescriptions Month and date