We often see news stories about the "biggest" drugs based on sales, however this way of looking at drug sales is somewhat skewed by the high cost of some therapeutics, particularly biologics. It is also noteworthy that the majority of the drugs are indicated for cancer.

Drug	Indication	Worldwide Sales 2018
Humira	Rheumatoid Arthritis	$19.936 billion
Eliquis	Anticoagulant	$9.872 billion
Revlimid	Cancer	$9.685 billion
Opdivo	Cancer	$7.570 billion
Keytruda	Cancer	$7.171 billion
Enbrel	Rheumatoid Arthritis	$7.126 billion
Herceptin	Cancer	$6.981 billion
Avastin	Cancer	$6.847 billion
Rituxan	Cancer	$6.750 billion
Xarelto	Anticoagulant	6.589 billion
Eylea	Cancer/AMD	$6.551 billion
Remicade	Crohn's Disease	$5.908 billion
Prevnar 13	Pneumonia	$5.802 billion
Stelara	Psoriasis	$5.156 billion
Lyrica	Epilepsy/Pain	$4.970 billion

I can't help but think that a better metric might be the number of patients treated. Whilst I don't have access to worldwide prescriptions the NHS in the UK does provide some information as part of the Prescription cost analysis for 2018. Whilst the number of prescriptions does correspond exactly with the number of patients treated I suspect it gives a very good indication.

Looking at the categories of drugs it is interesting to note that cancer does not figure in the top 20 categories. As you might expect lipid-lowering drugs, gastric ulcer treatment, treatments for cardiovascular disease, anti-depressants and analgesics are the most prescribed.

Drug Category	Number of items
Lipid-Regulating Drugs	74,289,246
Proton Pump Inhibitors	60,024,837
Angiotensin-Converting Enzyme Inhibitors	44,159,042
Calcium-Channel Blockers	41,102,081
Beta-Adrenoceptor Blocking Drugs	38,617,728
Selective Serotonin Re-Uptake Inhibitors	38,216,924
Non-Opioid Analgesics And Compound Prep	35,998,332
Antiplatelet Drugs	34,139,843
Thyroid Hormones	32,253,535
Control Of Epilepsy	27,989,893
Vitamin D	24,559,380
Opioid Analgesics	23,393,193
Selective Beta(2)-Agonists	22,900,805
Biguanides	21,806,787
Corticosteroids (Respiratory)	20,879,110
Angiotensin-II Receptor Antagonists	20,499,156
Oral Anticoagulants	17,613,257
Tricyclic & Related Antidepressant Drugs	16,704,980
Other Antidepressant Drugs	15,911,182
Thiazides And Related Diuretics	14,628,130
Antihistamines	13,626,254

Looking at the top most prescribed drugs, small molecule drugs dominate for all indications.

Chemical Name	Indication	Items
Atorvastatin	Lipid-Regulating Drugs	41,820,664
Levothyroxine Sodium	Thyroid Hormones	32,187,950
Omeprazole	Proton Pump Inhibitors	31,038,076
Amlodipine	Calcium-Channel Blockers	29,052,338
Ramipril	Angiotensin-Converting Enzyme Inhibitors	28,605,025
Lansoprazole	Proton Pump Inhibitors	25,461,167
Simvastatin	Lipid-Regulating Drugs	24,303,261
Bisoprolol Fumarate	Beta-Adrenoceptor Blocking Drugs	23,625,562
Colecalciferol	Vitamin D	23,609,903
Aspirin	Non-Opioid Analgesics And Compound Prep	23,397,042
Metformin Hydrochloride	Diabetes	21,806,787
Salbutamol	Selective Beta(2)-Agonists	21,577,054
Paracetamol	Non-Opioid Analgesics And Compound Prep	18,516,491
Co-Codamol (Codeine Phos/Paracetamol)	Opioid Analgesics	15,179,951
Sertraline Hydrochloride	Selective Serotonin Re-Uptake Inhibitors	14,815,719
Citalopram Hydrobromide	Selective Serotonin Re-Uptake Inhibitors	14,136,645
Amitriptyline Hydrochloride	Tricyclic & Related Antidepressant Drugs	13,532,567
Furosemide	Thiazides And Related Diuretics	11,945,445
Beclometasone Dipropionate	Corticosteroids (Respiratory)	10,671,698

Also of note is the number of prescriptions for drugs that are readily available over the counter.