and blood flow. Cholestasis leads to impaired fat absorption with deficiencies of fat‐soluble vitamins and impairment of absorption of lipophilic drugs. Alcoholic liver disease is common and chronic ethanol abuse is associated with increased activity of the microsomal ethanol‐oxidising system. This effect is a result primarily of induction by ethanol of a specific cytochrome P450 (CYP2E1) responsible for enhanced oxidation of ethanol and other P450 substrates and, consequently, for metabolic tolerance to these substances. This may lead to enhanced clearance and, hence, decreased response to certain drugs such as benzodiazepine sedatives, anticonvulsants (phenytoin) and warfarin. By contrast, simultaneous alcohol ingestion may decrease clearance of drugs metabolised via the P450 (CYP2E1) enzyme system.
Comment. Unlike the measurement of creatinine clearance in renal disease, there is no simple test that can predict the extent to which drug metabolism is decreased in liver disease. A low serum albumin, raised bilirubin and prolonged prothrombin time give a rough guide.
The fact that a drug is metabolised by the liver does not necessarily mean that its pharmacokinetics is altered by liver disease. It is not easy, therefore, to extrapolate the findings from one drug to another. This is because superficially similar metabolic pathways are mediated by different forms of cytochrome P450.
The documentation of modestly altered pharmacokinetics does not necessarily imply clinical importance. Even normal subjects show quite wide variations in pharmacokinetic indices and therefore pharmacokinetics should not be viewed in isolation from alterations in drug effect, which are much more difficult to assess. However, if a drug is known to be subject to substantial pharmacokinetic changes, clinical significance is much more likely.
If it is clinically desirable to give a drug that is eliminated by liver metabolism to a patient with cirrhosis, it should be started at a low dose and the drug levels or effect monitored very closely.
Altered drug effect deranged brain function
The more severe forms of liver disease are accompanied by poorly understood derangements of brain function that ultimately result in the syndrome of hepatic encephalopathy. However, even before encephalopathy develops, the brain is extremely sensitive to the effects of centrally acting drugs and a state of coma can result from administering normal doses of opiates or benzodiazepines to such patients.
Decreased clotting factors
Patients with liver disease show increased sensitivity to oral anticoagulants. These drugs exert their effect by decreasing the vitamin K‐dependent synthesis of clotting factors II, VII, IX and X. When the production of these factors is already reduced by liver disease, a given dose of oral anticoagulant has a greater effect in these patients than in subjects with normal liver function.
Worsening of metabolic state
Drug‐induced alkalosis
Excessive use of diuretics can precipitate encephalopathy. The mechanism involves hypokalaemic alkalosis, which results in conversion of NH4 + to NH3, the un‐ionised ammonia crossing easily into the central nervous system to worsen or precipitate encephalopathy.
Fluid overload
Patients with advanced liver disease often have oedema and ascites secondary to hypoalbuminaemia and portal hypertension. This problem can be worsened by drugs that cause fluid retention, e.g. NSAIDs, and antacids that contain large amounts of sodium. NSAIDs should be avoided anyway, because of the increased risk of gastrointestinal bleeding.
Hepatotoxic drugs
Where an acceptable alternative exists, it is wise to avoid drugs that can cause liver damage (Table 1.7), e.g. sulphonamides or rifampicin, and repeated exposure to halothane anaesthesia.
Table 1.7 Drugs that can cause liver damage.
| Hepatitis |
| Halothane (repeated exposure) |
| Isoniazid |
| Rifampicin |
| Methyldopa |
| Phenelzine |
| Trimipramine |
| Desipramine |
| Carbimazepine |
| Trasidone |
| Propylthiouracil |
| Augmentin |
| Erythromycin |
| Nitrofurantoin |
| Chloroguanide |
| Tienilic acid |
| Dihydralazine |
| Azothiaprine |
| Sulfasalazine (sulphasalazine) |
| Naproxen |
| Amiodarone |
| Cholestasis with mild hepatic component |
| Phenothiazines |
| Carbamazepine |
| Tricyclic antidepressants |
| Non‐steroidal anti‐inflammatory drugs (especially phenylbutazone) |
| Rifampicin, ethambutol, pyrazinamide |
| Sulphonylureas, trimethoprim |
| Sulphonamides, ampicillin, nitrofurantoin, erythromycin estolate |
| Oral contraceptives (stasis without hepatitis) |
| Cirrhosis |
| Methotrexate |
Prescribing for the young and the elderly
Prescribing for the young
A 4‐year‐old girl presents with her mother to the emergency department with shortness of breath, wheeze and a temperature. Her past medical history includes premature birth at 34 weeks gestation and asthma. She is prescribed salbutamol inhaler as required at home, which she has been using more frequently in the last 2 days. On examination, she is breathless at rest, is pyrexial, tachycardic and has bilateral rhonchi. She weighs 15 kg. A diagnosis of moderate acute exacerbation of asthma is made. You are asked to prescribe an anti‐pyretic medication, salbutamol and prednisolone.
Introduction
Prescribing
