Common Skin Conditions and How to Tell Them Apart
Skin conditions look alike more often than they behave alike. A red itchy patch can be eczema, a fungal infection, psoriasis, an allergic reaction or something considerably more serious, and the treatment for one of those makes another substantially worse.
This guide covers the common conditions, what distinguishes them, which treatments belong to which, and — most importantly — the small number of appearances that mean seeking help the same day rather than trying a cream.
🧠 The mistake that causes most of the confusion: putting a steroid cream on a fungal infection. The steroid suppresses inflammation, so the redness and itch improve for a few days and the rash appears to be responding — while the fungus, no longer restrained by the immune response, spreads. The result looks unlike ordinary ringworm, spreads over a wide area, and takes far longer to treat. It has a name, tinea incognito, and it happens constantly.
🔍 The questions that narrow it down
Dermatology relies on pattern recognition rather than tests, and a few questions do most of the work.
| Question | What it tells you |
|---|---|
| Where is it | Flexures suggest atopic eczema; extensor surfaces and scalp suggest psoriasis; between the toes and in skin folds suggests fungal |
| Does it itch | Intense itch points to eczema, scabies, urticaria; psoriasis itches less; painful rather than itchy suggests infection |
| Is it symmetrical | Symmetry suggests a systemic or constitutional cause; one-sided asymmetry suggests infection or contact |
| How fast did it appear | Minutes to hours suggests urticaria or irritant reaction; days suggests allergic contact dermatitis or infection; weeks to months suggests a chronic condition |
| Are there blisters | Small grouped blisters suggest herpes; larger ones suggest severe contact reaction or a blistering disease |
| Does it scale | Thick silvery scale suggests psoriasis; fine scale with a raised advancing edge suggests fungal |
| Anyone else affected | Household members itching suggests scabies |
🧴 Eczema and dermatitis
Eczema is not one condition but a pattern of inflammation with several causes, and the commonest is atopic eczema — a constitutional condition beginning in childhood, associated with asthma and hay fever, and driven by a defective skin barrier.
The barrier is the key concept. In atopic eczema the skin loses water more readily and lets irritants and allergens in more easily, which is why treatment is built around restoring it.
Emollients are the foundation and are consistently under-used. Applied generously and frequently — several times daily, and continued when the skin looks clear — they reduce flares, reduce the amount of steroid needed and relieve itch. Ointments work better than creams and creams better than lotions, at the cost of feeling greasier. Fragrance-free is essential.
Topical corticosteroids treat active inflammation, matched to the site:
| Site | Potency |
|---|---|
| Face, eyelids, genitals, skin folds | Mild only — thin skin absorbs readily |
| Trunk, arms, legs | Moderate |
| Hands, feet, thickened chronic patches | Potent — palms and soles resist absorption |
Use enough — the fingertip unit, the amount squeezed onto the last segment of an adult index finger, covers about two flat adult palms. Under-application is extremely common and is the main reason a correctly chosen steroid appears not to work. Anxiety about steroid creams leads to under-treatment, which prolongs the eczema and results in more total steroid use over time.
Where skin is broken, weeping or infected, a combined steroid and antibacterial such as betamethasone with gentamicin is used for a short course. Scratched eczema becomes infected readily, and treating inflammation alone in that situation does not clear it.
Other forms include contact dermatitis, covered in detail in the guide to contact dermatitis; seborrhoeic dermatitis, affecting the scalp, eyebrows, sides of the nose and chest, driven by a yeast that normally lives on skin and treated with antifungal shampoos and mild steroid; and discoid eczema, appearing as coin-shaped patches, often on the legs.
🩹 Psoriasis
Psoriasis affects around two per cent of people and is frequently mistaken for eczema or for a fungal infection.
What distinguishes it: thick, silvery, well-demarcated scaling plaques, typically on elbows, knees, scalp and lower back — the extensor surfaces, opposite to where atopic eczema sits. It itches less than eczema, and scratching the scale off leaves pinpoint bleeding.
Two things about psoriasis are widely unknown and both matter.
It is not just a skin condition. It is a systemic inflammatory disease, associated with higher rates of cardiovascular disease, metabolic syndrome, diabetes and depression. Cardiovascular risk factors should be assessed in anyone with moderate or severe psoriasis rather than treating the skin in isolation.
Around one in five develops psoriatic arthritis, which can be joint-damaging and which sometimes appears before the skin disease. Joint pain and stiffness, particularly morning stiffness lasting more than half an hour, swollen fingers or toes, and heel or lower back pain should be reported, since early treatment prevents permanent damage.
Treatment runs from topical vitamin D analogues and steroids, through phototherapy, to systemic drugs including acitretin and methotrexate, and biologic treatments for severe disease that have transformed outcomes over the past two decades.
It is not contagious, it is not caused by poor hygiene, and it flares with stress, infection, skin injury, smoking, alcohol and some medications including beta blockers and lithium.
🧫 Acne
Acne involves four processes together: increased oil production under hormonal influence, blocked follicles, overgrowth of a bacterium that normally lives on skin, and inflammation. Effective treatment addresses more than one of them, which is why single-agent approaches disappoint.
| Severity | Approach |
|---|---|
| Mild, mainly blackheads and whiteheads | Topical retinoid such as tretinoin, or benzoyl peroxide |
| Mild to moderate inflammatory | Topical retinoid combined with benzoyl peroxide or a topical antibiotic |
| Moderate | Oral antibiotic such as doxycycline alongside topical treatment; combined hormonal contraception in women |
| Severe, nodular, or scarring | Isotretinoin |
Three practical points determine results.
Topical retinoids irritate before they help. Redness, dryness and peeling in the first weeks are expected, and an initial worsening of spots is common. Starting every other night, applying a pea-sized amount to the whole area rather than to individual spots, and using a moisturiser reduces it. Most people who abandon retinoids do so during this phase.
Antibiotics are never used alone in acne and are not used long term. They are combined with benzoyl peroxide or a retinoid, which reduces resistance, and courses are limited in duration.
Treat scarring early. Scars are permanent and considerably harder to treat than active acne. Nodular or scarring acne warrants escalation rather than another course of the same thing.
🔴 Isotretinoin is highly effective and requires a specific safety framework. It causes severe birth defects, so anyone who could become pregnant needs reliable contraception, pregnancy testing before, during and after treatment, and must not become pregnant for a period after finishing. Liver function and lipids are monitored, mood changes are asked about, and it must never be shared or taken from someone else supply. It also should not be combined with tetracycline antibiotics. This is not a drug to obtain or take without that framework in place.
On the myths: chocolate and greasy food do not cause acne, though high-glycaemic diets and possibly skimmed milk show modest associations. Acne is not caused by poor washing — over-washing and scrubbing make it worse by damaging the barrier and increasing inflammation. And squeezing lesions increases the likelihood of scarring.
🍄 Fungal infections
Fungal skin infections have a characteristic appearance: a ring with a raised, scaly, advancing edge and relative clearing in the centre, spreading outward slowly. They are asymmetrical, and they itch.
Sites determine the name: the feet, the groin, the body, the scalp, and the nails, which are the most difficult to treat.
Most respond to topical antifungal treatment used for the full recommended period, which is usually longer than the rash lasts — stopping when it looks clear is the main reason it returns. Scalp and nail infections require oral treatment, since topical agents do not penetrate adequately.
The point made at the top of this article bears repeating here: steroid creams applied to fungal infections make them worse while briefly appearing to help. Any rash that improved on a steroid and then spread or changed character should be reconsidered as fungal.
🐛 Scabies
Scabies causes intense itch, characteristically worse at night, with a rash in the finger webs, wrists, armpits, waistline, genitals and around the nipples. Burrows may be visible as short wavy lines.
Three points determine whether treatment works.
Everyone in the household is treated at the same time, whether or not they itch, because the interval between infestation and symptoms is weeks and untreated contacts reinfest everyone else.
Treatment is applied to the entire body from the neck down, including between fingers and toes, under nails and in the genital area, and left on for the specified period. Permethrin cream is the usual choice, and a second application after a week is standard.
Itch continues for weeks after successful treatment, because it is an allergic response to the mites rather than evidence they are still present. This is the commonest reason people repeat treatment unnecessarily. Emollients and antihistamines help while it settles.
Bedding, towels and clothing used in the preceding days are washed hot or sealed in a bag for several days.
🧫 Bacterial infections
Impetigo produces golden-crusted lesions, usually around the mouth and nose, is highly contagious and mainly affects children. Localised cases are treated with a topical antibacterial such as mupirocin; extensive cases need oral antibiotics.
Boils and abscesses are collections of pus. The definitive treatment is drainage rather than antibiotics, though antibiotics are added where there is surrounding infection or systemic upset.
Cellulitis is infection of the deeper skin and the tissue beneath, producing a spreading area of redness that is warm, tender and often accompanied by fever and feeling unwell. It requires oral or intravenous antibiotics depending on severity — typically an anti-staphylococcal penicillin or a cephalosporin such as cephalexin.
A note on terminology, since it causes confusion: cellulitis is a serious bacterial infection; cellulite is dimpled fat on the thighs and is not a disease at all. The two words are frequently confused in translation and they have nothing to do with each other.
⚠️ Cellulitis needs same-day assessment where the redness is spreading, where there is fever or feeling generally unwell, where it involves the face or is near the eye, in anyone with diabetes or reduced immunity, or where pain seems out of proportion to what is visible. That last feature is important: severe pain with relatively unimpressive skin changes, particularly with rapid progression, can indicate a deep infection that requires emergency surgical assessment.
🔴 Appearances that need urgent attention
Most rashes are benign. A short list is not, and recognising them is the most valuable part of this article.
| Sign | Concern |
|---|---|
| A rash of small red or purple spots that does not fade when pressed with a glass | Can indicate meningococcal infection or a bleeding disorder — emergency assessment |
| Blistering with sore eyes, mouth or genitals, with fever, often after a new medication | Severe drug reaction such as Stevens-Johnson syndrome — emergency |
| Widespread rash with fever, facial swelling and feeling unwell weeks after starting a drug | Severe drug hypersensitivity reaction — emergency |
| Rapidly spreading redness with severe pain out of proportion to appearance | Deep soft tissue infection — emergency |
| A mole that is changing in size, shape or colour, is asymmetrical, has an irregular border, more than one colour, or is larger than about six millimetres | Possible melanoma — urgent assessment |
| A sore that has not healed in weeks, or a lump that bleeds and scabs repeatedly | Possible skin cancer — urgent assessment |
| A new pigmented streak under a nail | Possible melanoma — urgent assessment |
| Any rash with breathing difficulty, swelling of the lips or tongue, or faintness | Anaphylaxis — emergency, adrenaline first |
On moles specifically, the most useful single indicator is change. A mole that has looked the same for twenty years is far less concerning than one that has altered over months, and the one that stands out as different from all the others on the same person deserves attention regardless of whether it fits any checklist.
☀️ Sun protection
Ultraviolet exposure is the main modifiable cause of skin cancer and of photoageing, and the practical points are more specific than most advice suggests.
Sunscreen of adequate protection factor with broad-spectrum coverage, applied in a sufficient quantity — most people apply a fraction of what testing assumes, which substantially reduces the actual protection — and reapplied every couple of hours and after swimming.
Shade and clothing outperform sunscreen. Avoiding the hours around midday matters more than the product used.
Sunburn in childhood raises melanoma risk particularly, and sunbeds raise it substantially at any age.
Anyone taking a photosensitising medication — doxycycline, some diuretics, isotretinoin, certain antipsychotics and others — burns considerably faster than usual and should be told so.
❌ Things that make rashes worse
Steroid on a fungal infection, as above.
Antibiotic creams used repeatedly, which drive resistance and cause contact allergy — neomycin in particular is a frequent sensitiser.
Topical antihistamine creams, which work poorly and cause contact allergy themselves.
Over-washing and scrubbing, which damages the barrier in both eczema and acne.
Stopping a steroid too early out of fear, which prolongs the flare and results in more steroid overall.
Using someone else prescription, since the same appearance can require opposite treatments.
Fragranced products of any kind on inflamed skin.
Assuming a normal test result means nothing is wrong, since most skin diagnoses are made by examination rather than by test.
✅ The most useful habit for anyone with a persistent skin problem: photograph it. Skin conditions change between the day something is noticed and the day an appointment happens, and a dated photograph of the rash at its worst is more informative than any description. It also makes it possible to tell whether treatment is working, which is genuinely difficult to judge from memory alone.
🧑⚕️ When to see someone
- Anything in the urgent list above, at the timescale stated
- A rash not improving after a couple of weeks of appropriate treatment
- A rash that worsens on treatment, which suggests the diagnosis or the treatment is wrong, or an allergy to the product itself
- Widespread involvement, or the skin becoming generally red and scaly all over
- Signs of infection — increasing pain, spreading redness, crusting, pus or fever
- Significant effect on sleep, work or mood, which is a legitimate reason on its own and is under-reported
- Joint pain alongside psoriasis
- Any child with a rash who is unwell, rather than a rash alone
Treatment for the conditions described here is stocked in the skin care category — isotretinoin and tretinoin for acne, acitretin for psoriasis, permethrin for scabies, mupirocin for impetigo and a topical corticosteroid combination for infected dermatitis. Which applies depends entirely on the diagnosis, since several of these worsen the conditions the others treat.
Fungal infections of the nails, or of large areas of skin, that do not clear with creams are usually treated with tablets. Lamisil (terbinafine) is the standard choice for fungal nails, and a liver blood test is often checked before a long course.
❓ Frequently asked questions
Why did the steroid cream make my rash worse?
Most likely because it was a fungal infection. Steroids suppress inflammation, so redness and itch improve for a few days while the fungus, no longer restrained by the immune response, spreads. The result looks unlike ordinary ringworm and takes far longer to treat. Any rash that improved on a steroid and then spread or changed character should be reconsidered as fungal. Allergy to the cream itself is the other explanation.
How do I tell eczema from psoriasis?
Location and appearance. Atopic eczema favours the flexures — inside elbows, behind knees — itches intensely, and has poorly defined edges. Psoriasis favours extensor surfaces such as elbows and knees plus the scalp and lower back, produces thick well-demarcated silvery scaling plaques, and itches less. Scratching psoriasis scale off leaves pinpoint bleeding, which eczema does not do.
Are steroid creams dangerous?
Not when matched to the site and used for a defined course. Skin thinning and stretch marks come from potent steroids used continuously over long periods on thin skin, not from proper treatment. Anxiety about them causes under-treatment, which prolongs the condition and results in more total steroid over time. Use enough — the fingertip unit covers about two flat adult palms — and match potency to the site.
Why do emollients matter so much in eczema?
Because atopic eczema is fundamentally a barrier problem — skin that loses water readily and lets irritants and allergens in. Emollients restore that barrier, and applied generously and frequently, including when the skin looks clear, they reduce flares, reduce the amount of steroid needed and relieve itch. Ointments work better than creams and creams better than lotions. They are consistently the most under-used part of treatment.
Is psoriasis only a skin condition?
No, and this is widely unknown. It is a systemic inflammatory disease associated with higher rates of cardiovascular disease, metabolic syndrome, diabetes and depression, so cardiovascular risk should be assessed rather than treating the skin alone. Around one in five develops psoriatic arthritis, which can damage joints permanently and sometimes appears before the skin disease, so joint pain and morning stiffness should be reported.
Why did my acne get worse when I started the cream?
Topical retinoids commonly cause redness, dryness, peeling and an initial worsening of spots during the first weeks before improving. Most people who abandon them do so during exactly that phase. Starting every other night, applying a pea-sized amount across the whole area rather than to individual spots, and using a moisturiser reduces it considerably. Give it several weeks before judging.
Does diet or washing cause acne?
Chocolate and greasy food do not, though high-glycaemic diets and possibly skimmed milk show modest associations. Poor washing does not cause it either — over-washing and scrubbing make it worse by damaging the barrier and increasing inflammation. Squeezing lesions increases the likelihood of permanent scarring, which is considerably harder to treat than the acne itself.
What does isotretinoin require?
A specific safety framework, because it causes severe birth defects. Anyone who could become pregnant needs reliable contraception, pregnancy testing before, during and after treatment, and must not become pregnant for a period after finishing. Liver function and lipids are monitored and mood changes asked about. It must never be shared or taken from someone else supply, and should not be combined with tetracycline antibiotics.
I treated scabies but I am still itching. Did it fail?
Probably not. Itch continues for weeks after successful treatment because it is an allergic response to the mites rather than evidence they are still present, and this is the commonest reason people repeat treatment unnecessarily. Emollients and antihistamines help while it settles. What does cause genuine failure is not treating everyone in the household simultaneously, or not applying the cream to the entire body.
What is the difference between cellulitis and cellulite?
They are unrelated despite the similar words, which are frequently confused in translation. Cellulitis is a bacterial infection of the deeper skin producing spreading warm tender redness, often with fever, and requiring antibiotics. Cellulite is dimpled fat on the thighs and buttocks, is not a disease, and requires no treatment. Only one of them is a medical condition.
Which mole changes should worry me?
Change itself is the most useful indicator — a mole that has looked the same for twenty years is far less concerning than one that has altered over months. Beyond that: asymmetry, irregular borders, more than one colour, diameter over about six millimetres, and any mole that stands out as different from all the others on the same person. A new pigmented streak under a nail also warrants urgent assessment.
Which rashes are emergencies?
A rash of small red or purple spots that does not fade when pressed with a glass. Blistering with sore eyes, mouth or genitals and fever, particularly after a new medication. Widespread rash with fever and facial swelling weeks after starting a drug. Rapidly spreading redness with pain out of proportion to what is visible. And any rash with breathing difficulty, lip or tongue swelling, or faintness.
📑 Sources and editorial
- British Association of Dermatologists guidelines on eczema, psoriasis, acne and skin infections
- NICE guidance on atopic eczema in children, psoriasis assessment and management, and acne vulgaris
- Evidence on psoriasis as a systemic inflammatory condition and its cardiovascular associations
- Literature on tinea incognito and corticosteroid-modified fungal infection
- Guidance on isotretinoin pregnancy prevention programmes and monitoring requirements
- Fingertip unit methodology for topical corticosteroid quantification
- Melanoma early detection criteria and the ugly duckling sign
- Related reading: contact dermatitis
- Related products: skin care category
- RXshop Editorial Team — reviewed by Emily Chen, MD, MPH, Internal Medicine
Medical Disclaimer: The information in this article is for educational and informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek guidance from a qualified healthcare provider with any questions you may have regarding a medical condition, and before starting, stopping or changing any medication.