Lash and Brow Serum: Prostaglandins, Peptides or Castor Oil — What Actually Works?

The market for lash and brow serums is growing by double-digit percentages every year, with manufacturers promising "300% thicker lashes in 4 weeks". The reality is more nuanced. In this article we examine the four main categories of active ingredients — from the most potent (and most controversial) to those that have more marketing than science behind them.

Prostaglandins and their analogues: the only proven "growth boosters"

Prostaglandin analogues are the only group of ingredients with hard clinical evidence for lengthening and thickening lashes. Their discovery was accidental — patients using bimatoprost eye drops for glaucoma noticed that their lashes were becoming longer and darker.

Bimatoprost (Latisse)

Bimatoprost is a prescription drug (in Poland: Lumigan, Latisse). It is the only ingredient approved by the FDA (2008) for the treatment of eyelash hypotrichosis. Clinical trials have shown:

• An average 25% increase in lash length after 16 weeks
• A 106% increase in thickness
• An 18% increase in darkness

But there is a flip side. The side effects of bimatoprost are real and well documented:

• Iris darkening (hyperpigmentation) — in people with mixed eye colours (green-brown, blue-grey), the brown pigment can intensify permanently. The effect is irreversible.
• Loss of orbital fat (prostaglandin-associated periorbitopathy, PAP) — long-term use can cause the eye to appear "sunken", deepen the eyelid sulcus and create dark shadows. The phenomenon has been described in the ophthalmology literature since 2004.
• Discolouration of the eyelid skin — reversible once you stop using the product.

Isopropyl cloprostenate (the cosmetic analogue)

Because bimatoprost requires a prescription, cosmetics manufacturers use "cosmetic" prostaglandin analogues, the most popular of which is isopropyl cloprostenate. It works on the same principle — it activates the FP prostaglandin receptor, extending the anagen (growth) phase of the lash.

The problem: there are no clinical trials of the same quality as those for bimatoprost. The efficacy is plausible, but the side effects may be identical — sunken orbits and changes in iris colour have also been reported with cosmetic analogues. Check your serum's ingredient list in the PurScore ingredient database to identify any prostaglandin analogues.

Peptides: a gentler alternative with promising data

Peptides are short chains of amino acids that can stimulate hair growth without the prostaglandin mechanism — and therefore without the risk of changing eye colour or causing fat atrophy.

Myristoyl Pentapeptide-17

The most studied peptide in the context of lashes. An in vitro study (published in the International Journal of Cosmetic Science) showed that it stimulates keratin expression in hair follicles, which translates into thicker and stronger lashes. Clinical studies (though smaller than those for bimatoprost) show:

• A 10-15% increase in lash length after 8 weeks
• No significant side effects
• Reversible effects once you stop use (as with all growth-stimulating substances)

Biotinoyl Tripeptide-1

Combines biotin with a tripeptide, making it easier to penetrate the hair follicle. The clinical data are limited, but the mechanism of action makes sense — biotin is a cofactor for carboxylase enzymes, which are key to keratin synthesis.

Peptides are a sensible compromise between efficacy and safety. The effects are less dramatic than with prostaglandins, but the risk is minimal.

Topical biotin: weak evidence

Biotin (vitamin B7, vitamin H) is widely marketed as "the vitamin for hair and lashes". What does the science say?

• Oral biotin helps only in cases of a documented deficiency — which is extremely rare in the general population. A 2017 systematic review (Skin Appendage Disorders) found no evidence that supplementation is effective in people without a deficiency.
• Topical biotin has even weaker evidence. The biotin molecule is hydrophilic and penetrates the skin barrier poorly. There are no controlled clinical trials confirming that biotin in a lash serum translates into measurable growth.

Biotin in a lash serum's formula is more of a marketing ingredient than an active one. It won't do any harm, but you shouldn't base your expectations on it.

Castor oil (Castor Oil): anecdotes vs evidence

Castor oil is probably the oldest "home remedy" for lashes and brows. There are thousands of anecdotal accounts online. What does the science say?

There is not a single controlled clinical trial confirming that castor oil lengthens or thickens lashes. The only published study (Syed et al., 2019) concerned scalp hair only and showed that ricinoleic acid (the main component of castor oil, 85-90%) has anti-inflammatory properties that may support healthy hair follicles — but that is not the same as stimulating growth.

What castor oil actually does:

• Moisturises and coats the lashes, reducing brittleness
• Adds shine, which visually "thickens" them
• Can cause contact allergy in sensitive individuals

Castor oil is a good emollient, but it is not a growth-stimulating substance. If you're looking for real results, peptides or (under a doctor's supervision) prostaglandins are a better choice.

How to read a lash serum's ingredient list

What to look for in the INCI:

1. Look for the active ingredient in the first half of the list — if the peptide is at the end, there's too little of it to work.
2. Identify hidden prostaglandins — names such as isopropyl cloprostenate, dechloro dihydroxy difluoro ethylcloprostenolamide, or methylamido dihydro noralfaprostal are all prostaglandin analogues.
3. Check the preservatives — a serum applied to the lash line must be sterile; phenoxyethanol is a good choice.

Not sure what's hiding in your serum's ingredient list? Search for the product on PurScore — we'll automatically identify prostaglandin analogues, peptides and potential allergens. You can read more about sunscreens and how they interact with eye makeup in our guide to SPF.