> ## Documentation Index > Fetch the complete documentation index at: https://docs.hi-doctor.ai/llms.txt > Use this file to discover all available pages before exploring further. # Side effects: dermatology topicals > Retinoid purge and irritation, benzoyl peroxide bleaching fabric, hydroquinone ochronosis, azelaic acid stinging, and why daily sun protection is not optional. Almost every side effect of a dermatology topical is local, and most of them are worst in the first few weeks. That pattern is the reason so many people stop a treatment that would have worked: the medicine is at its most unpleasant exactly when it has produced no visible benefit yet. This page separates the effects that settle from the ones that do not, and the ones that mean stopping from the ones that mean waiting. ## The retinoid adjustment phase Tretinoin and adapalene both accelerate epidermal turnover in skin that is not used to it. The first four to six weeks are called retinisation, and the experience is consistent: dryness, flaking, tightness, stinging, redness, and often an apparent worsening of the skin. **Purge versus failure.** A retinoid pushes lesions that were already forming beneath the surface to mature faster, so more of them appear at once early on. That is the purge, and it settles. A treatment failure looks different: new lesions still appearing at the same rate after eight to twelve weeks of consistent use, with no reduction in the rate of formation. The purge is a timing effect at the start; failure is a rate that has not changed by the end. What reduces retinisation, in order of how much difference it makes: 1. **Apply to completely dry skin.** Damp skin increases penetration and increases irritation. Waiting after washing is the single most effective adjustment. 2. **Reduce the frequency.** Applying less often at the start and building up is the standard approach, and the schedule is set by the reviewing doctor. 3. **Buffer with a bland moisturiser**, applied before or after the retinoid. 4. **Stop the other actives.** Acids, scrubs, vitamin C and exfoliating devices are the most common reason retinisation becomes intolerable. Irritation that is spreading, blistering, weeping, intensely itchy or accompanied by facial swelling is not retinisation. That is a reason to stop and seek advice. ## Benzoyl peroxide bleaches fabric, permanently Benzoyl peroxide oxidises dye. It bleaches towels, pillowcases, bedding, clothing, hair, beards and eyebrows, and the colour is destroyed rather than stained — nothing washes it out. Use white towels and white bedding, let the product dry fully before dressing or going to bed, and keep it off the hairline and brows. This is entirely preventable, and it is the most common avoidable complaint about the medicine. Benzoyl peroxide also causes dryness, peeling, stinging and redness, mostly in the first fortnight and mostly settling. A true allergic contact dermatitis to it is uncommon but well documented, and it looks different from ordinary irritation: intensely itchy, spreading beyond where the product was applied, and getting worse rather than better. ## Hydroquinone and ochronosis **Exogenous ochronosis** is a paradoxical blue-black or grey-brown discolouration of skin treated with topical hydroquinone. It is associated with prolonged, uninterrupted use — particularly at higher concentrations, over long periods, and without sun protection. It is difficult to treat and can be permanent. It is the reason hydroquinone is prescribed in defined courses with breaks rather than continuously, and the reason the melasma questionnaire asks specifically about your previous hydroquinone exposure. The practical rule that follows: the course length is set by the reviewing doctor, and continuing past it or restarting without review is exactly the behaviour the rule exists to prevent. Hydroquinone also causes stinging, dryness and irritant dermatitis, and it can produce halo hypopigmentation — a visibly lighter ring of normal skin around the treated patch, more likely when the product is applied beyond the area it was meant for. ## Azelaic acid stings The burning or tingling sensation on applying azelaic acid is the single most commonly reported effect of the medicine. It is not an allergy and not a sign the strength is wrong — it is a characteristic of the molecule, it is strongest in the first week or two, and it settles as tolerance develops. Applying to completely dry skin and starting at a reduced frequency are what reduce it. Dryness, scaling and mild redness are also common early. Hypopigmentation is reported rarely and is worth watching for on darker skin. Worsening of asthma has been reported rarely, and is worth mentioning if you have asthma. ## Photosensitivity, and why sun protection is non-negotiable Every one of the four dermatology pathways here converges on the same requirement, for four different reasons. | Pathway | Why daily broad-spectrum protection is part of the treatment | | ---------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | [Acne](/library/conditions/acne) | Retinoids and benzoyl peroxide both increase ultraviolet reactivity, and sunlight darkens the post-inflammatory marks acne leaves behind — which are usually what people mind more than the spots | | [Rosacea](/library/conditions/rosacea) | Sunlight is among the most frequently reported flare triggers, and one of the few that can be controlled reliably | | [Melasma](/library/conditions/melasma) | Ultraviolet **and** visible light both drive melanogenesis. Treating pigment while exposing it to light is applying the stimulus faster than the medicine removes the result. Tinted, iron-oxide-containing protection is what covers the visible-light part | | [Skin ageing](/library/conditions/skin-ageing) | Photoageing is the damage being treated. Continuing to accumulate it while treating it is self-defeating, and a retinoid raises ultraviolet reactivity on top of that | In practice: broad-spectrum, high factor, applied every morning and reapplied through the day, on cloudy days, through windows, and in winter. If only one element of a routine survives, this is the one that should. ## What each medicine is most likely to do | Medicine | Most likely early effect | The one to watch for | | --------------------------------------------------------------------------- | ------------------------------------------ | ------------------------------------------------------------- | | [Tretinoin](/library/medicines/tretinoin) | Retinisation — dryness, flaking, stinging | Pregnancy exclusion; photosensitivity | | [Adapalene with benzoyl peroxide](/library/medicines/adapalene) | Retinisation plus benzoyl peroxide dryness | Pregnancy exclusion; permanent fabric bleaching | | [Benzoyl peroxide](/library/medicines/benzoyl-peroxide) | Dryness and peeling | Allergic contact dermatitis; fabric and hair bleaching | | [Clindamycin with benzoyl peroxide](/library/medicines/clindamycin-topical) | Local irritation from the peroxide | Severe, persistent or bloody diarrhoea — stop and seek advice | | [Azelaic acid](/library/medicines/azelaic-acid) | Stinging on application | Hypopigmentation on darker skin; rare asthma worsening | | [Hydroquinone](/library/medicines/hydroquinone) | Stinging and irritation | Ochronosis with prolonged use; halo hypopigmentation | | [Metronidazole](/library/medicines/metronidazole-topical) | Mild dryness or stinging | A rosacea flare that begins after starting it | | [Ivermectin](/library/medicines/ivermectin-topical) | Burning or dryness early | A transient early flare that does not settle | | [Niacinamide](/library/medicines/niacinamide) | Usually nothing | Rarely the cause when a compounded formula is not tolerated | ## Pregnancy Topical retinoids — tretinoin and adapalene — are **not used in pregnancy**, when trying to conceive, or while breastfeeding. Hydroquinone is avoided in pregnancy and breastfeeding. The melasma combination formula contains both, so it is excluded on both counts. Every dermatology questionnaire has a pregnancy step, and it is a hard gate. The reviewing doctor decides on every other medicine on this page, and needs to know before prescribing rather than after. ## Stop and seek advice if * the irritation is spreading beyond where you applied the product, blistering, weeping or intensely itchy * your face, lips, tongue or throat swell, or you have any difficulty breathing — this is emergency care, not a message * you develop severe, persistent or bloody diarrhoea while using a clindamycin-containing product * the treated area is getting darker rather than lighter, or has taken on a blue-grey cast * a rosacea flare started after a new product and has not settled * there is no change at all after twelve weeks of consistent use Messages to the doctor go through the private chat in your account. The review is asynchronous throughout — there is no video call and no telephone consultation. ## Related reading * [Acne](/library/conditions/acne) · [rosacea](/library/conditions/rosacea) · [melasma](/library/conditions/melasma) · [skin ageing](/library/conditions/skin-ageing) * [How to apply a prescription topical](/library/guides/how-to-apply-topicals) * [Taking your dermatology photos](/library/guides/dermatology-photos) * [Safety and eligibility](/essentials/safety) · [how a consultation works](/essentials/how-it-works) ## Start a consultation Four pathways: [acne](https://hi-doctor.ai/consultation/dermatology-acne/1), [rosacea](https://hi-doctor.ai/consultation/dermatology-rosacea/1), [melasma](https://hi-doctor.ai/consultation/dermatology-melasma/1) and [anti-ageing](https://hi-doctor.ai/consultation/dermatology-antiaging/1). A registered doctor reviews each one and declining is a normal outcome. See [what it costs](/essentials/pricing) and [how the review works](/essentials/how-it-works). This page is reference information, not medical advice and not a prescription. Whether a medicine is appropriate for you is decided by a registered doctor after reviewing your consultation.