Monster vs Red Bull vs Relentless: Caffeine, Sugar and Price Compared

Monster, Red Bull and Relentless are three of the biggest names in the UK energy drink aisle — and three of the most searched-for comparisons. On the surface they do the same job, but the differences in caffeine, sugar, can size and price per can are bigger than most people realise. Here’s how they actually stack up, so you can pick the right case for your fridge.

The Quick Answer

If you want the strongest hit per can, Monster and Relentless are tied — both deliver 160mg of caffeine in a 500ml can. Red Bull’s classic 250ml can contains 80mg, which is the same strength per 100ml but half the total dose. On sugar, Relentless is the lightest of the three full-sugar options, Red Bull sits in the middle, and original Monster carries the most. On price, Relentless is consistently the best value per can when bought in bulk.

Caffeine Content Compared

Caffeine is the reason most of us reach for an energy drink, so it’s the obvious place to start.

Notice the pattern: all three brands are formulated at the same strength — 32mg per 100ml, which is the practical ceiling for mainstream energy drinks sold in the UK. The real difference is the can. A 500ml Monster or Relentless is simply a double dose compared with a standard 250ml Red Bull.

For context, a single espresso contains roughly 60–80mg of caffeine, so one large can is broadly equivalent to two espressos. The European Food Safety Authority advises healthy adults to stay under 200mg in a single sitting and 400mg across a day — so one 500ml can gets you close to half your daily allowance in one go. If you’re sensitive to caffeine, the smaller Red Bull 250ml can is the more forgiving choice.

Sugar Content Compared

This is where the three brands genuinely part ways.

Monster — the sweetest of the three

Original green Monster contains around 11g of sugar per 100ml, which works out at roughly 55g per 500ml can — well above the NHS recommended daily maximum of 30g of free sugars for adults. That’s a big part of why it tastes the way it does, and why the energy hit feels so immediate. If you love the Monster flavour without the sugar load, the Ultra range is the answer: Monster Ultra Peachy Keen is zero sugar with a near-identical caffeine kick.

Red Bull — middle of the pack

A 250ml Red Bull contains about 27g of sugar — nearly seven teaspoons, and most of an adult’s daily free-sugar allowance in one small can. Per 100ml it’s similar to Monster; the smaller can is what keeps the total down. A sugar-free version exists if you want the classic taste without it.

Relentless — the reformulated one

Relentless was reformulated after the UK’s Soft Drinks Industry Levy came in, and now contains roughly 24g of sugar per 500ml can — under 5g per 100ml, which keeps it below the levy threshold. That makes Relentless Cherry the lowest-sugar full-flavour option of the three: half the sugar of a Monster in the same size can, with the same 160mg of caffeine.

Price Per Can: Which Is Cheapest?

Single cans from a corner shop typically run £1.50–£2.00 or more. Buying by the case changes the maths completely. Based on our current 12-pack prices:

• Relentless — from £11.99 per 12 pack, which works out at around £1.00 per 500ml can
• Red Bull — from £14.99 per 12 pack, roughly £1.25 per 250ml can
• Monster — from £15.99 per 12 pack, roughly £1.33–£1.42 per 500ml can depending on flavour

On pure value — millilitres and milligrams per pound — Relentless wins comfortably. Monster costs more per can but delivers the widest flavour range by far, from Juiced Pipeline Punch to Aussie Style Lemonade. Red Bull holds its price because, frankly, it can — it remains the benchmark taste that everything else is measured against.

Taste and Flavour Range

Taste is subjective, but the broad strokes are consistent. Red Bull is sharp, light and slightly medicinal — a clean, familiar profile that hasn’t changed in decades, now joined by fruit Editions like Tropical and Apricot-Strawberry. Monster is sweeter and heavier, with the biggest flavour catalogue in the category across the Original, Ultra and Juiced lines. Relentless sits between the two: smoother and less sweet than Monster since its reformulation, with a compact range built around a few strong fruit flavours.

If you genuinely can’t decide, that’s what our variety packs are for — a mix of flavours from one brand in a single case, so you can find your favourite before committing to twelve of the same.

So Which Should You Buy?