Pre-Workout vs Pump Formula: Stim or Stim-Free, Which Should You Choose?
Quick answer: A stim pre-workout contains caffeine; a pump formula is stim-free and built around ingredients such as citrulline, beta-alanine and betaine. Choose a stim pre-workout for morning or afternoon sessions if you handle caffeine well; choose a pump formula for evening training, if you are sensitive to caffeine, or if you have already had caffeine that day. You can stack the two, but caffeine should come from one product only and the combined beta-alanine adds up fast.
Stim vs stim-free: what is the difference?
- Stim pre-workout: contains caffeine, sometimes alongside other ingredients. Example: a scoop with 300mg or more of caffeine plus citrulline and beta-alanine.
- Stim-free or pump formula: no caffeine or other stimulants. Usually built around citrulline, which researchers think works by raising nitric oxide production. Example: a scoop with 10g of citrulline and 0mg of caffeine.
- Pump: the temporary, fuller feeling in a working muscle during higher-rep training, caused by extra blood flow into it.
The biggest difference you will feel is the caffeine. A 2018 review of multi-ingredient pre-workouts concluded that caffeine appears to be the main ingredient behind several of their short-term effects.
PRE V4 vs PUMP V2: side by side
Both labels list every ingredient and dose. Prices are standard UK RRP and can change.
| Per serving | DARKSTIMS PRE V4 | DARKSTIMS PUMP V2 |
|---|---|---|
| Type | Stim pre-workout | Stim-free pump formula |
| Caffeine | 362.5mg (from a 400mg dual-source caffeine matrix) | 0mg |
| Citrulline | 8,000mg L-citrulline | 10,000mg (8,000mg L-citrulline + 2,000mg citrulline nitrate) |
| Beta-alanine | 3,500mg | 5,000mg |
| Betaine anhydrous | Not included | 4,000mg |
| L-tyrosine | 2,000mg | 3,000mg |
| CDP choline | 800mg | 1,000mg |
| L-theanine | 200mg | Not included |
| Servings per tub | 25 (20g each, 500g tub) | 20 (550g tub) |
| RRP | £35 (£1.40 per serving) | £40 (£2.00 per serving) |
What the research says about the key ingredients
The studies below tested individual ingredients, not either of these products.
- Caffeine: the ISSN position stand concludes caffeine consistently improves exercise performance at 3–6mg per kg of body mass, with small to moderate benefits for muscular endurance, strength, sprinting and jumping in many but not all studies.
- Citrulline: a 2019 meta-analysis of 12 studies found a small but significant benefit for high-intensity strength and power outcomes (effect size 0.20). A 2021 meta-analysis found 6–8g of citrulline malate taken 40–60 minutes before training increased repetitions to failure by about 6% on average. Our L-citrulline dosage guide goes deeper.
- Beta-alanine: the ISSN says 4–6g a day for at least 2–4 weeks has been shown to improve exercise performance, with the clearest effects in efforts lasting one to four minutes. Its only reported side effect is tingling (paraesthesia).
Betaine, L-tyrosine and CDP choline also feature in these formulas; this guide sticks to the ingredients with the strongest published reviews.
Which should you choose?
| If you… | Choose | Why |
|---|---|---|
| Train in the morning or early afternoon and tolerate caffeine well | Stim pre-workout | A 2018 review linked caffeine to most of a pre-workout's short-term effects |
| Train in the evening | Stim-free pump formula | In one study, caffeine taken six hours before bed (400mg) still cut sleep by over an hour |
| Are new to caffeine or sensitive to it | Stim-free, or a low-caffeine product | EFSA's single-dose safety figure is 200mg |
| Already drink two or more coffees a day | Stim-free, or count carefully | EFSA's daily figure is 400mg from all sources |
| Train twice a day | Stim for one session, stim-free for the other | Keep to one caffeinated serving in 24 hours |
Stim pre-workout: pros and cons
- Pros: caffeine is the ingredient most linked to a pre-workout's short-term effects, with consistent performance research behind it; you feel it working.
- Cons: the benefit shrinks with daily use as tolerance builds; it can cost you sleep; it adds to your daily caffeine total; not for under-18s, pregnant or breastfeeding women, or anyone caffeine sensitive or with a medical condition.
Stim-free pump formula: pros and cons
- Pros: no caffeine, so no caffeine-related sleep disruption and nothing to add to your daily total; usable for late sessions.
- Cons: no stimulant lift; research effects for citrulline are small; in our range it costs more per serving.
Can you stack pre-workout and a pump formula?
Yes. Stacking gives you caffeine from the pre-workout and extra citrulline from the pump formula. Caffeine stays at one product's dose, but the other ingredients add up:
| Ingredient | PRE V4 + full PUMP V2 | PRE V4 + half PUMP V2 |
|---|---|---|
| Caffeine | 362.5mg | 362.5mg |
| Citrulline (total) | 18,000mg | 13,000mg |
| Beta-alanine | 8,500mg | 6,000mg |
| Betaine anhydrous | 4,000mg | 2,000mg |
| L-tyrosine | 5,000mg | 3,500mg |
| CDP choline | 1,800mg | 1,300mg |
A full serving of each puts 8,500mg of beta-alanine into one drink, above the 4–6g daily range the ISSN describes. The ISSN also notes tingling can be reduced with smaller, divided doses, so expect more tingle from the full stack. A full PRE V4 with half a PUMP V2 keeps beta-alanine at 6,000mg.
Never stack two caffeinated products. PRE V4 with ULTRA V2, or PRE V4 with Caffeine Electrolytes (512.5mg combined), takes a single session past EFSA's 400mg daily figure.
How the DARKSTIMS range fits
- DARKSTIMS PRE V4: 362.5mg caffeine per serving (from a 400mg dual-source caffeine matrix), 25 servings, £35 RRP or £28 on Subscribe & Save. For experienced, healthy adults; half a scoop the first time.
- DARKSTIMS PUMP V2: zero caffeine, 10,000mg citrulline, 20 servings, £40 RRP.
- DARKSTIMS ULTRA V2: 500mg caffeine per serving, for healthy adults 18+ already used to high-caffeine products only, not for daily use.
- DARKSTIMS stacks: bundles priced below buying the same items separately.
FAQ
Is a stim-free pre-workout worth it?
It depends on when you train. Without caffeine you lose the ingredient most responsible for a pre-workout's short-term effects, but you keep citrulline and beta-alanine, which have their own research behind them, and you protect your sleep. For evening training it is usually the better trade.
Can I take PRE V4 and PUMP V2 together?
Yes. Take no more than one serving of PRE V4 in 24 hours and no other caffeine with it. Combining full servings gives 8,500mg of beta-alanine; a full PRE V4 with half a serving of PUMP V2 keeps it to 6,000mg.
Will I feel a stim-free pre-workout?
Not in the way you feel caffeine. You may notice a tingling sensation from beta-alanine, which the ISSN lists as its only reported side effect. Our guide to the beta-alanine tingle explains why it happens.
Can I take a pump formula in the evening?
Yes. A stim-free formula like PUMP V2 contains no caffeine, so it does not carry caffeine's sleep cost. See when to take pre-workout for timing and how much caffeine is in pre-workout for daily caffeine limits.
Sources
- Guest NS et al. (2021). International Society of Sports Nutrition position stand: caffeine and exercise performance. JISSN
- EFSA (2015). EFSA explains risk assessment: Caffeine (summary of the Scientific Opinion on the safety of caffeine)
- Harty PS et al. (2018). Multi-ingredient pre-workout supplements, safety implications, and performance outcomes: a brief review. JISSN
- Trexler ET et al. (2019). Acute effects of citrulline supplementation on high-intensity strength and power performance: a systematic review and meta-analysis. Sports Medicine
- Vårvik FT et al. (2021). Acute effect of citrulline malate on repetition performance during strength training: a systematic review and meta-analysis. IJSNEM
- Trexler ET et al. (2015). International Society of Sports Nutrition position stand: beta-alanine. JISSN
- Drake C et al. (2013). Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. Journal of Clinical Sleep Medicine
DARKSTIMS products are food supplements and should not be used as a substitute for a varied, balanced diet and a healthy lifestyle. Do not exceed the stated serving. Speak to your GP before use if you are pregnant, breastfeeding, under 18, taking medication or have a medical condition. Caffeinated products are not suitable for children, pregnant or breastfeeding women, or anyone sensitive to caffeine; count caffeine from all sources and do not combine caffeinated products.
