We’ve heard good things about this fat burner and so we tasked our nutritionist James Dixon with testing and delivering this Phentaslim review – find out all of his thoughts on it below…

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Phentaslim is one of the best thermogenic fat burners on the market. Though it shares characteristics with dozens of similar products, it does so with more elan, more style, and greater efficacy than most. Today, we want to put it up against the competition and find out just how good it is as part of this Phentaslim review.

It promises a range of benefits, all of which will help your weight loss efforts, often in unexpected ways. But can you expect better results, more efficiently achieved, for less effort, stress and worry, when you combine it with a good quality calorie deficit and active lifestyle?

Quick Verdict: Phentaslim

Phentaslim is a good example of its type. Though perhaps a little standard looking, it looks to combine the best of what other products offer.

You will burn through fat a little more easily whilst finding it easier to stick to your eating plan, but be mindful that each serving is very high in caffeine (which won’t be suitable to all).

There are a couple of better thermogenic fat burners out there, our favourite being PhenQ which you should consider over Phentaslim and contains half the amount of caffeine per serving.

About Phentaslim

Phentaslim is a standard, yet very good quality, very intelligently designed thermogenic fat burner, brought to us from OptimumNutra.

OptimumNutra have been going for over a decade now, which is a long old time in a marketplace as changeable and fast-paced as the supplements industry. In that time, they have developed a reputation for the purity and potency of their products.

This shows with Phentaslim. It is one of the best examples of its type – it is a high quality, very useable fat burner. There is little to make it stand out too drastically from its competition, but it does what it does very well. It gives you all the benefits you would expect, including increased calorie burn, increased energy levels, improved cognitive function and mental wellbeing, and a little appetite suppression.

There are better products out there, but not many. I tried it for myself and was incredibly happy with the results I got. More than this, I was very happy with the ease and efficiency with which I achieved these results – it was one of the most pleasant, effective calorie deficits I’ve ever run.

How Phentaslim Works

Phentaslim works like many of its competitors – it improves your body’s metabolic function, allowing you to burn more calories at rest (around 50-100 daily, which is no small thing), whilst giving you a good energy kick, cutting through the brain fog and lethargy associated with restricted eating plans, and leading to mild, natural appetite suppression.

Phentaslim bottle

In short, it makes every element of your weight loss diet easier to achieve and more effective.

As you’ll see below, it includes some potent ingredients to make this happen. Together, they should increase rates of thermogenesis by a significant degree. Any weight loss regime rises or falls based on your overall calorie intake. To lose 1 lb per week (0.5 kg), you need a daily deficit of about 500 calories. This can come from a mixture of diet (eating less) and extra calorie burn (being more active). It can also come, in small part, from increased thermogenesis.

In burning an extra 50-100 calories per day, a supplement like Phentaslim can give you upwards of 10% of your deficit essentially for free. Alternatively, it can give you an extra boost to a solid deficit, increasing it and thus optimising your weight loss. I would typically suggest you go with the latter option.

Our metabolisms also often suffer during a calorie deficit. They slow down, making you sluggish and stopping you from being able to burn through fat at the same rate as before. In optimising your metabolism using a thermogenic fat burner, you are ensuring that your deficit actually allows you to lose weight, all whilst mitigating many of its negative effects.

This is backed up by a good energy kick, courtesy of plenty of caffeine, among other things.

This caffeine content should also act as a mild appetite suppressant. There are better supplements out there than Phentaslim for appetite suppression, but it should still give you some benefits.

Where Phentaslim really excels is in its inclusion of a few nootropic ingredients, which I’ll get into below. These should benefit your cognitive health and mental wellbeing. Together, they will mitigate the effects of stress and anxiety, which is incredibly important in any weight loss context, whilst keeping you alert and happy, with the creation of a good kick of extra neurotransmitters.

This is a formidable list of benefits, on par with some of the best thermogenic fat burners on the market. Though you shouldn’t expect miracle weight loss from taking anything like Phentaslim (you’ll still need to put in the work and stick to a good calorie deficit!), the whole dieting process will be easier, more pleasant, and more effective for taking it.


Phentaslim makes use of a who’s who of decent thermogenic and weight loss specific ingredients. Arguably, this makes it a little vanilla – there is little in here that sets it apart from its competition. However, there is no arguing with its formulation. It’s a solid list of top hits.

You get a good energy boost from each serving. This is due to the combination of guarana seed and caffeine. They are both profound thermogenic ingredients able to boost your metabolism even as they cut through the lethargy and brain fog so often associated with restricted eating plans. There is also evidence suggesting that caffeine can aid with appetite suppression.

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Caffeine can be hit or miss in a supplement like this.

Coffee lovers like me tend to get on very well with it, as you would expect. Some are sensitive to it, however.

Some struggle with the dramatic peaks and crashes it delivers. Guarana delivers caffeine in a slightly different form, however – it is slower release, steadier, known for its ability to deliver increased energy without the heart stopping jitters associated with other forms.

The caffeine content comes from caffeine anhydrous, one of the most potent forms going.

It’s a good one-two with which to begin.

This thermogenesis is backed up by capsicum rich cayenne pepper. Capsicum offers many health benefits. However, in this context, it’s included for its thermogenic properties. It is one of the most potent thermogenic compounds going. Combining it with decent caffeine levels will realistically result in as large a thermogenic effect as it’s possible to get from a supplement.

Phentaslim also includes Panax Ginseng. This is an odd one, though it sort of makes sense from a couple of angles. There is no evidence to suggest that it can help with weight loss. However, it serves two functions. It is good for mental and cognitive health, which will generally be very welcome throughout a weight loss diet.

It has also been associated with increased natural testosterone production. Testosterone levels in men are intimately connected with healthy body composition, making it a good inclusion for male users.

Magnesium and zinc, Phentaslim’s next ingredients, serve a similar purpose. Both have been shown to raise both total and free levels of testosterone, making them good dietary aids for men. Magnesium is also required for your body’s energy production, which may then give you a bit of an athletic boost. 

Magnesium is also known for its laxative effects. This may help to keep your digestive health on point during a calorie deficit as restricted eating plans typically contain suboptimal fibre levels.

Next up, we have green tea extract. Green tea offers many great benefits, including an extra caffeine hit, and thus an extra touch of increased thermogenesis.

Green tea is also rich in l-theanine, which you’ll often see in nootropic supplements.

L-theanine is known for its ability to aid relaxation and mitigate the symptoms of stress and anxiety. As stress and anxiety are common causes of overeating, this is a very clever addition. You will have a slightly higher rate of thermogenesis from the green tea; you’ll also be far more relaxed and far less likely to binge eat.

This is backed up by the inclusion of l-tyrosine, another common nootropic ingredient. L-tyrosine is essential in the formulation of several neurotransmitters (chemicals in your brain), including epinephrine, norepinephrine, and dopamine. Collectively, these will lead to greater happiness and overall wellbeing paired with lower levels of stress and anxiety.

You also get a decent vitamin B complex with Phentaslim. This includes vitamins B6, B2, B1 and B12. Collectively, these lead to increased amino acid (protein) absorption and cell growth and function, meaning greater muscle growth and athleticism, and energy metabolization.

Finally, Phentaslim includes black pepper extract, which is something of a must in this kind of supplement (or any supplement, really). It is a thermogenic compound in itself, albeit not in the same league as caffeine or capsicum.

However, it makes everything else in any given supplement far more bioavailable: it’s known for its ability to improve the body’s nutrient absorption. Your body will be able to make better use of everything else in Phentaslim’s formula because of it.

Does Phentaslim Work?

Yes, Phentaslim works well. For me to say this, unequivocally, a thermogenic fat burner of its kind needs to do three things.

Firstly, it needs to elicit greater energy burn. This is hard to measure. However, the science is sound. I maintained an average 500 calorie daily deficit whilst taking it and lost slightly more than the weekly 1 lb (or 0.5 kg) that I was expecting. Over a four week period, I lost just over 5 lbs (I hit 2.5 kg lost). This leads me to believe Phentaslim’s thermogenic claims quite happily.

Secondly, it needs to suppress your appetite. This isn’t Phentaslim’s strongest point. For this, I would recommend something like Phen24 that includes a good dose of glucomannan. However, the caffeine intake works well in theory. I certainly found myself less tempted to snack. My calorie deficit came a lot more easily than usual.

Phentaslim before and after
Before and after pictures when using Phentaslim according to the manufacturer

Finally, it needs to give you a good energy boost. I’m perhaps not the right person to test this. I drink a lot of coffee (a writer’s prerogative) and so have a decent tolerance to caffeine. I’m usually loaded up with it as a matter of course. However, I found myself drinking less coffee than usual whilst still being able to hammer out a couple of thousand words per day and make it to the gym for a good workout.

If you’re more caffeine sensitive than me, then you might want to try something like Phen24 which contains around half the amount of caffeine.

Phentaslim also works well as a nootropic. I typically don’t suffer too much with brain fog (or, rather I do, but I drink coffee to get through it!) However, I do get a little anxious, a little stressed, as deadlines loom and the caffeine takes its toll. This will always lead to a greater temptation to overeat, especially for emotional eaters.

I found myself calmer on Phentaslim than I usually would be, either in general or, specifically, during a calorie deficit.

It’s a little tricky to take Phentaslim, though it’s far from the worst supplement going for clever timings. You should take one capsule three times daily, 20-30 minutes before each meal. However, I would suggest taking one before a mid-afternoon snack rather than waiting for dinner time – that caffeine content will likely interrupt your sleep patterns if you take it much later than 3pm.

There are few side effects associated with Phentaslim. All the ingredients are natural and safe. However, that caffeine content may get to you – it may make you jittery, slightly nauseous, and may lead to tension and headaches if you’re sensitive to it.

Evidence To Support Phentaslim

There is little evidence to support Phentaslim’s effects as a whole, other than user testimonial (of which there is plenty). However, there is a lot of evidence supporting its individual ingredients, lending it a solid foundation of scientific justification.

For instance, there is a lot of evidence supporting both caffeine and cayenne pepper for boosting metabolism.

A recent, seminal study in The American Journal of Clinical Nutrition found that cayenne pepper was good for raising body temperature (eliciting thermogenesis), thus increasing your metabolic rate.

A recent study in Clinical and Experimental Pharmacology and Physiology showed that caffeine could increase the fuel cost of activity and alter muscle thermogenesis, heightening metabolism and energy output.

A study published in Medicine and Science in Sports and Exercise looked into tyrosine, finding that it could significantly improve cognitive behaviour alongside athletic performance, making it a perfect supplement for those looking to lose weight whilst staying physically active and mentally alert.

Meanwhile, green tea has been studied extensively for its use in weight loss. Many of its effects can be traced back to its caffeine content, of course. However, the catechins it delivers have been linked to further weight loss, increasing fat burning both during and post- exercise. The antioxidants it gives you have been linked to greater fat mobilisation from cells, in addition.

The formula is clever. Though untested in its entirety, OptimumNutra have based their ingredients list on well-known, cutting-edge research. Everything is data led and scientifically justified.


I wouldn’t necessarily recommend you go with an alternative. Phentaslim is well thought out and effective. My one gripe with it is that it’s slightly vanilla – its ingredients list, whilst very workable, is very similar to every other good quality thermogenic fat burner’s.

It is also extremely high in caffeine.

And if you’re going to go with something a bit vanilla, you may as well aim for the best. In this case, this means PhenQ, the industry gold standard for thermogenic fat burners.

It gives you all the benefits you would expect of a good thermogenic fat burner – increased energy use, decreased appetite, increased energy, greater clarity, and so on. However, it does so with a little more… style, efficiency and competency!

It works the same as any other fat burner, just with greater potency and efficiency, with better quality, better dosed ingredients, and better customer care, than pretty much any other supplement.

By all means, give Phentaslim a go. I got some great results from it and would be happy to recommend it to anyone. Just don’t forget that it’s a crowded marketplace. Don’t forget that there are stellar products like PhenQ and Phen24 out there.


As I said at the start, there is nothing incredibly special about Phentaslim, but that doesn’t matter. It doesn’t need to reinvent the wheel. It just needs to make the wheel to a high standard, which it really, really does.

Sure, there are better products out there, but only two or three. And it’s a crowded marketplace – making the top five is a hell of an achievement.

If you take Phentaslim as part of a weight loss diet and exercise plan, you will benefit more than you are likely expecting to. Your brain will be clearer, you’ll be happier, and you’ll be calmer. This has a profound knock-on effect, allowing you to lose weight more effectively without the angst and worry that often both comes with and undermines your efforts.

You will also be able to lose more weight, more easily, without any of the sluggishness generally associated with a calorie deficit.

This is everything and more that a thermogenic fat burner should offer – Phentaslim do a wonderful job of ticking every box.