A third pneumonia-causing drug is the powerful anticancer agent methotrexate, also widely used in low doses against arthritis and severe psoriasis. It is thought to work by interfering with cell growth and suppressing the immune system. In about one in 20 patients, the drug has been found to attack the lungs, causing a range of conditions, including pneumonia.

What has surprised researchers is the rapidity and virulence of the drug’s side-effects. Pneumonia can be triggered after as little as two months of therapy, and by doses as low as 5 mg a week (Rev Rhum Engl Ed, 1996; 63: 453-6). Furthermore, the pneumonia may be irreversible, so that even stopping the methotrexate will not sort out the problem. A significant number of cases end in death, even with the very low doses (15 mg/week for a month) used for psoriasis (Mil Med, 2004; 169: 298-300).

Acne is another relatively minor skin condition that can also lead to pneumonia if treated by some prescription drugs. The antibiotic minocycline (Minocin) is a common treatment for acne, but it, too, can cause “various pulmonary complications”, including pneumonia characterised by “relapsing acute respiratory failure” (Chest, 2003; 123: 2146-8).

Why drugs cause pneumonia How is it that so many disparate drugs can cause such similar problems? According to French experts, there’s a wide range of explanations - from inflammation and swelling of lung tissue to outright lung cell poisoning (Rev Mal Respir, 1996; 13: 127-32).

In fact, it now turns out that the lungs are especially sensitive to a whole host of prescribed drugs - not just antibiotics, heart and arthritis medications, but also milder drugs such as antidepressants, appetite suppressants and even aspirin (see lower box). “Drug-induced lung disease is a major source of iatrogenic injury,” say doctors at the Department of Pulmonary Care in the Cleveland Clinic. “In 1972, only 19 drugs were known to cause pulmonary disease. Now, at least 150 agents are recognised, and the list continues to grow” (Cleve Clin J Med, 2001; 68: 782-5, 789-95).

This is disturbing enough, but now comes the news that lung disease may be even more dangerous than we thought. Experts from a number of UK hospitals voiced their concerns in a landmark analysis of the medical records of 40,000 patients. They found an association between respiratory problems such as pneumonia and a subsequent heart attack or stroke - in fact, a fivefold increase in risk.

What lung infections apparently do is disturb the fat/plaque buildup in arteries, turning a theoretical problem into a killer disease (N Engl J Med, 2004; 351: 2599-610).

Among the chief investigators was Professor Patrick Vallance of University College London, who put it like this: “After the age of 50, we all have some degree of furring up in the arteries, but most of the time, it sits there fairly harmlessly. However, during infection, stable deposits become unstable and may break off, causing blockages that may lead to a heart attack or stroke.”

This means that drugs that cause pneumonia as a side-effect may be more dangerous than we thought - even those seemingly benign (and profitable) antacids such as Tagamet and Zantac. Professor Vallance hasn’t yet made the connection to drug-induced pneumonia - but we have.

On studying the available published mortality statistics for the major antacid ulcer drugs, we’ve found a consistent pattern. As soon as people start taking these drugs, the death rate soars. For H2 antagonists such as cimetidine (Tagamet), the rate doubles to a 90 per cent increase in fatalities in the first year (Gut, 1992; 33: 1280-4). For the three major proton-pump inhibitors (omeprazole, esomeprazole and lansoprazole), there’s an overall 60 per cent increase in non-ulcer deaths (J Gastroenterol Hepatol, 2005; 20: 11-25); omeprazole (Losec) is associated with a 44 per cent increase in deaths in year one, some of which are ‘circulatory diseases’ (Gut, 2003; 52: 942-6).