Cimetidine reduces the metabolism of verapamil, diltiazem and nifedipine by blocking liver enzymes, but only the blood levels of nifedipine may be raised to a level where the interaction is clinically significant. Verapamil reduces the clearance of digoxin which can result in an increase in digoxin serum levels by an average of 70 per cent. The effect of diltiazem is to raise levels by 20 per cent, whereas nifedipine has little or no effect. There has been a huge increase in the availability of slow release formulations, particularly nifedipine, in recent years. They also serve to improve patient compliance and may offset the lack of benefits that have been identified when the drug is used in a non-modified form.
Potassium channels Potassium channel openers is a new class of cardiovascular drug.
It is a relatively new drug and its place in therapy is still evolving. The drug causes smooth muscle relaxation by selectively increasing the membrane permeability of potassium ions, thus causing the membrane to hyperpolarise. They can be effective in reducing the symptoms of angina and have few side-effects. In addition, there is no problem with tolerance, which occurs with nitrates. Formularies Increasingly, pharmacists are being asked to advise general practitioners on the development of formularies.