Contact Lenses Guide: Types, Fitting, Hygiene and Risks

Contact lenses are worn by well over a hundred million people worldwide. They give a wide, natural field of view without frames, do not fog up or slide down your nose, and allow freedom in sport and many jobs. For some eye conditions, such as keratoconus or very irregular corneas, specialist lenses provide better vision than glasses ever could. Modern materials are more breathable and comfortable than ever.

Contact lenses are also medical devices that sit directly on the living surface of the eye. Used correctly, they are very safe; used carelessly, they can lead to painful and sometimes sight-threatening infections. This guide explains the main types of lenses, what to expect at a professional fitting, the hygiene rules that matter most and the warning signs you should never ignore. If you are deciding between contact lenses, glasses and surgery, our page on refractive errors provides useful background.

How contact lenses work

A contact lens floats on the tear film covering the cornea, the clear front window of the eye. Like glasses, it corrects focusing errors such as myopia, hyperopia, astigmatism and presbyopia by adding or subtracting focusing power. Because the lens sits on the eye rather than about 12 mm in front of it, the prescription is often slightly different from your spectacle prescription, particularly above about plus or minus 4 dioptres, and the lens moves with your eye so your view is not limited by a frame.

The cornea has no blood vessels and gets most of its oxygen directly from the air through the tear film. Lens materials therefore need to let enough oxygen through. Silicone hydrogel materials, introduced in the late 1990s, transmit far more oxygen than older hydrogel lenses and are now widely used.

Soft contact lenses

Soft lenses are made of flexible, water-containing plastics. They are comfortable from the start and are the most widely worn type.

Daily disposables

You insert a fresh pair each morning and throw them away at night. There is no cleaning, no solution and no case, which removes several of the main sources of infection. Daily disposables are ideal for beginners, occasional wearers, children and teenagers, people with allergies and those who want maximum convenience. They are usually more expensive per day of wear than reusable lenses if worn every day, but can be economical for part-time use.

Two-weekly and monthly lenses

These reusable lenses are cleaned and stored overnight in a disinfecting solution and replaced on a set schedule. They are cheaper for daily wearers but require strict care. Wearing them beyond their replacement date increases deposits, discomfort and infection risk.

Extended and continuous wear

Some silicone hydrogel lenses are approved for sleeping in, sometimes for up to a week or a month. However, overnight wear increases the risk of corneal infection several-fold compared with daily wear, so this should only be done if your practitioner specifically recommends it and monitors you.

Lenses for astigmatism and presbyopia

Toric lenses

Astigmatism means the cornea or lens focuses light unevenly in different directions. Toric lenses have different powers in different meridians and a stabilising design, often a slightly heavier lower edge, so that they stay in the correct orientation when you blink. They are available as daily, two-weekly and monthly soft lenses and in rigid materials. If a toric lens rotates, vision may fluctuate; your practitioner may try a different design.

Multifocal and monovision lenses

From the mid-forties, presbyopia makes near vision difficult. Multifocal contact lenses contain zones of different powers, usually in concentric rings, so that the brain learns to select the clear image for each distance. Many people adapt well, though vision may be slightly less crisp than with glasses, especially in dim light. An alternative is monovision, where one eye is corrected for distance and the other for near. Some people combine approaches. A trial period is the best way to find out what works for you, and some people simply wear distance lenses with reading glasses over them.

Rigid lenses

Rigid gas permeable (RGP) lenses

RGP lenses are small, firm lenses, usually about 9 to 10 mm across, made of oxygen-permeable plastics. They hold their shape, so the tear layer beneath them smooths out irregularities on the cornea. This gives very sharp vision, especially for higher astigmatism and corneal irregularities. They are durable, often lasting a year or more, and resist deposits well. Their main drawback is initial comfort: most people need a few weeks of gradually increasing wear to adapt, and they can dislodge more easily during sport or when dust gets underneath.

Scleral lenses

Scleral lenses are large rigid lenses, typically 15 to 22 mm across, that vault completely over the cornea and rest on the white of the eye (sclera). The space between the lens and the cornea is filled with sterile saline at insertion, creating a fluid reservoir that keeps the cornea moist and masks irregularities. Scleral lenses are a major treatment option for keratoconus, corneal scarring, irregular corneas after surgery or transplant, and severe dry eye conditions. They are fitted by specialist practitioners and need careful handling, but many wearers find them surprisingly comfortable.

Orthokeratology (ortho-k)

Orthokeratology lenses are special rigid lenses worn overnight. They gently reshape the front surface of the cornea while you sleep, so that you can see clearly during the day without lenses or glasses. The effect is temporary and reverses within days if you stop wearing them. Ortho-k is used for mild to moderate myopia and is also one of the evidence-based options to slow the progression of myopia in children. Because the lenses are worn during sleep, meticulous hygiene and regular follow-up are essential to reduce infection risk. Our article on why myopia is rising discusses myopia control more broadly.

Coloured and cosmetic lenses

Coloured lenses can enhance or completely change eye colour, and special-effect lenses are popular for costumes and events. They can also be made with or without a prescription, and some prosthetic tinted lenses help people with damaged or discoloured eyes or light sensitivity from conditions such as aniridia.

Comparison of lens types

The contact lens fitting

A contact lens fitting is more than a vision test. It usually includes:

• A discussion of your needs, hobbies, work and any eye or general health conditions, including allergies and medications.
• An up-to-date refraction to determine your prescription.
• Measurement of the curvature and size of the cornea, often with a corneal topographer.
• Examination of the eyelids, tear film and cornea with a slit lamp microscope to check you are a suitable candidate. Significant dry eye or eyelid inflammation may need treatment first.
• Trial lenses to assess fit, movement, centration and vision.
• Training in insertion, removal and care, and a follow-up visit after a period of wear.

After the fitting you receive a contact lens specification that includes the lens brand, material, base curve, diameter and power. It is not the same as a glasses prescription. Regular aftercare checks, usually once a year or as advised, allow your practitioner to detect early problems such as oxygen-related changes or inflammation before you notice them. Contact lens wearers should also keep an up-to-date pair of glasses as a backup.

Hygiene: the rules that prevent infection

Most serious complications are linked to avoidable hygiene mistakes. The most important rules are:

1. Wash and dry your hands thoroughly before touching your lenses or eyes.
2. Keep lenses away from water: never rinse or store lenses or cases in tap water, and avoid swimming, showering or using hot tubs in lenses. Water can carry Acanthamoeba, a microorganism that causes a rare but very serious corneal infection. If you swim in lenses, use tight-fitting goggles and discard daily lenses afterwards.
3. Use fresh solution every time: never top up old solution, and use only the products recommended by your practitioner.
4. Rub and rinse reusable lenses with solution if instructed, even with no-rub solutions.
5. Look after the case: empty it, rinse with solution, air-dry face down with caps off, and replace it at least every three months.
6. Follow the replacement schedule: do not stretch the life of your lenses.
7. Do not sleep in lenses unless prescribed for overnight wear.
8. Never use saliva to wet a lens or share lenses with others.
9. Apply make-up after inserting lenses and remove lenses before removing make-up.
10. If in doubt, take them out: remove lenses whenever your eyes are uncomfortable or red.

For a more detailed list of common errors, read our article on contact lens hygiene: 12 mistakes to avoid.

Risks and complications

Most contact lens wearers have no serious problems, but it is important to recognise the possible complications:

• Microbial keratitis: an infection of the cornea, usually bacterial and occasionally caused by Acanthamoeba or fungi. It is the most serious complication and can cause scarring and permanent vision loss. The main risk factors are overnight wear, poor hygiene, water exposure and smoking. See our page on corneal ulcers and eye infections.
• Dryness and discomfort: very common, especially with screen work, air conditioning and towards the end of the day.
• Giant papillary conjunctivitis: an allergic-type reaction under the upper eyelid causing itching, mucus and lens intolerance.
• Corneal hypoxia: lack of oxygen from over-wear or low-oxygen lenses can cause new blood vessels to grow into the cornea.
• Corneal abrasions: scratches from damaged lenses, debris or rough handling.

Who should be cautious or avoid contact lenses?

Contact lenses may not be suitable for people with severe dry eye, active eye infections or inflammation, poorly controlled allergies, or difficulty maintaining hygiene. People working in very dusty or chemical environments may need protective eyewear instead. Children can wear contact lenses successfully once they can handle them responsibly, often with daily disposables, under close professional supervision.

When to see a doctor

Book a routine appointment if lenses become uncomfortable, your vision changes, or you notice mild redness that settles when lenses are removed but keeps returning.