Patients often want to know whether their illness or the treatment they are receiving will affect their ability to drive. Specific advice must always be given by a doctor who knows the patient's medical history and nature of their illness but there are some general guidelines. First, it is always best to assume that anyone taking prescribed drugs should not drive.
One major problem is that many drugs can cause drowsiness. Obviously tranquillisers and sleeping tablets are likely to produce this particular problem. But the same effect can be caused by remedies prescribed for colds, coughs and allergies such as hay fever. For the first 48 hours after starting drug treatment, assume that you'll be unsafe. Then just wait and see how you feel after a couple of days. However, if your doctor or pharmacist warns you that sleepiness is a recognised problem with the drug you're taking, then you must avoid driving altogether.
Many articles taken from 'A word with the doctor', by Dr. John Windsor.