Comorbidity investigated (more than one condition can be shown per study)
| Disease | Number of studies |
| Multiple conditions investigated | 28 |
| Single conditions investigated | 12 |
| COPD and emphysema | 23 |
| Cardiovascular disease | 21 |
| Asthma | 16 |
| Diabetes | 14 |
| Gastro-oesophageal reflux disease, hiatus hernia and peptic ulcer | 12 |
| Cerebrovascular disease | 9 |
| Rheumatoid arthritis | 7 |
| Anxiety or depression | 7 |
| Malignancy | 6 |
| Osteoporosis and osteopenia | 6 |
| Peripheral vascular disease | 5 |
| Hyperlipidaemia | 4 |
| Chronic kidney disease | 3 |
| Rhinosinusitis | 2 |
| Tuberculosis | 2 |
| Liver disease | 2 |
| Pulmonary fibrosis | 1 |
| Pulmonary hypertension | 1 |
| Connective tissue disease | 1 |
| Immune deficiency | 1 |
| Allergic bronchopulmonary aspergillosis | 1 |
| Osteoarthritis | 1 |
| Pneumoconiosis | 1 |
| Dementia | 1 |
| Anaemia | 1 |
| Urinary incontinence | 1 |
| Non-tuberculous mycobacteria | 1 |
| Irritable bowel disease | 1 |