We're talking about a palliative treatment. Since we're not talking about arresting the disease process, but merely slowing it, then cost/benefit is a prime consideration. So far no studies have proved that the drug makes these patients more comfortable. An increase in vital capacity is good, if it translates into a lower work of breathing. That would produce subjective relief. So far I'm not seeing convincing evidence of subjective relief.
Pulmonary fibrosis causes ventilation/perfusion imbalance. It isn't helpful if ventilation improves in regions of the lung that are poorly perfused due to scarring. An improvement in exercise tolerance or in hemoglobin saturation would indicate an overall improvement in ventilation-perfusion ratios. The studies are not showing this result.
No drug interaction studies have been done, so it is risky to add a drug with poorly understood action to the regimen of patients with pulmonary fibrosis because many have other medical conditions. For example doxycycline, ibuprofen, and statin drugs are photo-toxic. Does Perfenidone potentiate this action?