Stewart House
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Pharmacy Expert Witnesses
Pharmacists are experts in how medicines work and their impact on the body relating to not only prescribed, but also bought medicines along with the mechanism of action of illegal drugs. Where harm is caused by inappropriate prescribing or inaccurate supply of medicines, pharmacists are ideally placed to quantify the impact and the likely harm.
Dr Richard Brown FRPharmS
Pharmacists are experts in how medicines work and their impact on the body relating to not only prescribed, but also bought medicines along with the mechanism of action of illegal drugs. Where harm is caused by inappropriate prescribing or inaccurate supply of medicines, pharmacists are ideally placed to quantify the impact and the likely harm.
However, there is more to a pharmacist beyond the safe and effective supply of medicines. The Pharmacy Degree covers the following:
• Mechanism of action of medicines on the body (pharmacokinetics)
• Processes of absorption, distribution, metabolism and excretion from the body
• Medicine formulation
• The impact of the method of action
• Injections
• Oral
• Other administration routes
• Law and legislation relating to medicines and drugs
• Human Medicines Regulations
• Misuse of Drugs Act
• Medicines Act
• Medication licencing and intended uses, including unlicenced indications
• Understanding of product licences and their specific characteristics
• Medicinal and analytical chemistry
This means that a pharmacist expert can provide an expert answer to questions relating to medicines that other professions may not have experience of.
Background on the properties of drugs and the action on the body
The life of a drug in the human body is characterised by the following pathway
(pharmacokinetics).
1. Absorption – process of the drug entering the body
2. Distribution – process of the absorbed drug moving around the whole body
3. Metabolism – process of changing the chemical make-up of the drug to make it
easier for the body to remove
4. Excretion – process of removing the drug from the body. Often this is via urine.
When a drug (medicine) is taken or administered into the body it begins a journey
characterised by the above stages of pharmacokinetics. For example, paracetamol.
1. You have a headache, you want the pain to go away, you therefore take the
paracetamol tablet and swallow it.
2. From your stomach onward the paracetamol tablet is beginning to break up and
Dissolve.
3. As it dissolves it enters the blood stream (absorption) and will become distributed
across the body where it will exert its desired affect.
4. However, at the same time the body is now starting the process of metabolism and
excretion to remove the drug from the body.
5. During this phase, absorption is happening at a faster rate compared to metabolism
and excretion enabling the concentration of paracetamol in the blood to increase.
6. Once all the paracetamol is absorbed the concentration in the blood stream will
have reached its maximum.
7. At some point the rising concentration of paracetamol will have reached a level to
exert a therapeutic benefit. In this instance, the paracetamol will be having an
effect at reducing the pain. The drug is now said to be in its therapeutic window.
8. Now that absorption is no longer occurring metabolism and excretion become
dominant and the blood levels begin to drop. At some point the blood levels will
drop below the concentration required to exert a therapeutic benefit and as a result
pain relief may stop. This may have resolved your headache, or you may need to
take more paracetamol. Whichever you decide the drug will either continue to be
excreted until removed, however absorption may become dominant again if you
take another tablet
The diagram below shows these concepts. In the first part of the graph (the upslope),
absorption is occurring faster than metabolism and excretion until the peak of effect is
reach. This is also called Cmax and occurs after a certain amount of time post administration
called Tmax. Time to Tmax is always important to note as it indicates how long the drug takes
to work.
The duration of action is the length of time the concentration of drug is high enough to
achieve the desired clinical response. In the case of pain relief, the length of time it keeps
you pain free. If you want to keep someone in the therapeutic window beyond the
duration of action you need to administer more medication. You will then tend to find that
the peak of effect is higher because you have the cumulative effect of the second dose
added to the residual concentration from the first dose. The risk here is you move above
the therapeutic window and into the adverse effects or side effects.
Metabolism and excretion combined work to eliminate the active drug from the body. The
speed of elimination is measured by the half-life of the drug and is this is the time taken to
remove half of the concentration of the drug from the body. This can be either through
metabolism to an inactive chemical or excretion from the body. For example:
Half-life of 2 hours
• Drug concentration at peak 100mg / L
• Drug concentration after two hours (one half-life) – 50mg / L
• Drug concentration after four hours (two half-lives) – 25mg / L
• Drug concentration after six hours (three half-lives) – 12.5mg / L
Most drugs are considered to have negligible effect after four to five half-lives
When things go wrong
As we all know things can go wrong, people and organisations make mistakes. When this happens harm can be caused. Sometimes, things go wrong without warning and systems learn, however in some occasions, lessons are not learnt and mistakes continue to occur. When this happens, people, organisations and systems are at fault and pharmacists are well trained to implement safe operating systems, which means when systems fail, they are also highly skilled in rationalising the failing and the impact.
Standard Operating Procedures (SOPs) and Clinical Governance should be at the heart of patient safety in all organisations. Sadly, and all too often, Senior Managers see this as getting in the way of patient care and therefore look to cut corners as they see this hindering clinical practice. However, this could not be further from the truth. Safe and effective clinical practice goes hand in hand with efficient, well run, safe organisations as safety is part of the culture rather than an afterthought. This means when teams are trained and supported, they can work quickly and safely to deliver high quality patient care. When it goes wrong, people openly reflect and learn from mistakes. If this culture doesn’t exist patient care is often compromised and organisations do not learn from mistakes. This leads eventually to significant corporate failings.