O2 Tablet: Uses, Benefits, Dosage & Side Effects

O2 Tablet is an antibiotic that contains ofloxacin 200 mg and ornidazole 500 mg. It is used to treat certain bacterial and parasitic infections, especially mixed infections.

Ofloxacin fights bacteria, while ornidazole works against certain bacteria and parasites such as amoeba and giardia.

O2 is available in different forms and strengths, so always check the medicine pack before taking it. It is a prescription medicine in India and should be taken only as advised by a doctor.

Ofloxacin blocks two enzymes inside bacteria, DNA gyrase and topoisomerase IV. Bacteria need them to copy and repair their DNA. With these enzymes blocked, the bacteria cannot multiply, and the medicine kills them.

Ornidazole is taken up by anaerobic bacteria and protozoa, and is converted into active forms only inside those organisms. Those active forms break the strands of the organism's DNA and stop it from being copied, which kills the organism. Human cells are not affected in the same way, which is why the medicine can target the infection.

Because the two work in different ways, the combination covers a wider range of organisms than either medicine alone.

O2 Tablet is used only if a doctor has prescribed it, for infections caused by organisms it can act on. The product information for this medicine lists these uses:

• Diarrhoea and dysentery caused by a bacterial or protozoal infection
• Pelvic inflammatory disease
• Infections after surgery, including prophylaxis (prevention) in planned surgery, after appendix removal, in intra-abdominal infections and in hernia repair
• Dental infections, such as root canal infections and dental abscesses
• Ear, nose and throat infections
• Skin and soft tissue infections

This is the Indian product information. In the United States and some other countries, regulators allow ofloxacin only for a narrower set of infections, and advise reserving it for patients who have no other treatment option. In India, whether it is right for you is your doctor's decision — not something to judge from a list.

What it is not for

• It does not treat viral illnesses.
• It is not a fever medicine. It does not bring a fever down quickly; it treats the infection behind it, if that infection is one it can act on.
• Many episodes of loose motions are viral in origin, and antibiotics do not help those. Using antibiotics when they are not needed is a major driver of antibiotic resistance, so let a doctor decide whether you need it.

When a doctor prescribes this combination, the aim is straightforward: to treat an infection that may involve more than one type of organism without adding a second prescription.

• Two actions in one tablet. Ofloxacin covers a range of bacteria, while ornidazole adds cover against anaerobic bacteria and protozoa.

Whether these benefits actually apply to you depends on the infection being caused by organisms the medicine can act on. Your doctor may also want a test before or during treatment.

Take it exactly as your doctor prescribes. The product information for this medicine states one tablet twice daily for adults, but your doctor decides your dose and how many days you take it, depending on what is being treated.

A few practical points:

1. Swallow the tablet whole with a full glass of water. Do not crush or chew it unless your doctor or pharmacist says it is fine.
2. Take one in the morning and one in the evening, about 12 hours apart, at the same times each day. You can take it with or without food - the antibiotic is absorbed either way. If it upsets your stomach, take it with a meal.
3. Keep a gap of at least two hours before and after antacids, iron, calcium or zinc supplements, sucralfate, and buffered didanosine tablets. These bind to ofloxacin in the gut and can stop it being absorbed properly.
4. Take it for exactly as long as your doctor advises. Do not stop early on your own, and do not extend the course. Finish the whole course your doctor prescribed, even if you feel better after a day or two.
5. Do not share these tablets with anyone else, and do not keep leftover tablets for a future illness.

Paediatric forms of this combination exist in India, but ofloxacin is not established as safe under 18, so they should be given only on a doctor's prescription — never an adult tablet to a child.

Most people who need this medicine tolerate it, but side effects can happen. Some are mild and settle; a few need urgent medical attention.

Common side effects include:

• Nausea and vomiting
• Stomach pain, cramps, gas or loose motions
• Headache
• Dizziness or light-headedness
• Trouble sleeping
• A metallic or bitter taste, or loss of appetite
• Rash or itching
• Vaginal itching or discharge in women

Get medical help straight away if you notice any of these:

• Pain, swelling or inflammation in a tendon - most often the Achilles tendon at the back of the heel, but also the shoulder, hand, thumb or biceps. It can happen within hours or days of starting, or even months after finishing.
• Tingling, burning, numbness, "pins and needles" or weakness in your hands or feet.
• Confusion, anxiety, hallucinations, severe headache that will not go away, or a seizure.
• Watery or bloody diarrhoea that does not settle, or comes on weeks after you stop the medicine.
• Yellowing of the eyes or skin, dark urine, or severe tiredness.
• A skin rash with blistering or peeling, or swelling of the lips, tongue or face, or trouble breathing.
• A fast or irregular heartbeat.
• Shakiness, sweating or confusion if you also take diabetes medicines.
• Sudden chest, stomach or back pain.

Nerve and brain effects with ornidazole are rare, but they have been reported even after short courses - one report describes a child who developed unsteady walking and slurred speech after 15 days of this combination. These problems usually improve once the medicine is stopped.

Do not take this medicine if any of the following applies to you:

• You have ever had an allergic reaction to ofloxacin, to any other quinolone or fluoroquinolone antibiotic, or to ornidazole or another nitroimidazole such as metronidazole or tinidazole.
• You have had tendinitis or a tendon rupture, or a tendon problem after taking any quinolone antibiotic.
• You have had nerve damage (peripheral neuropathy) after taking a quinolone.
• You have myasthenia gravis - ofloxacin can make muscle weakness and breathing problems worse.
• You are pregnant. Pregnancy is listed as a contraindication in the product information for this medicine.
• Your doctor has told you that you must not take it for any other reason.

Tell your doctor before starting if you are not sure whether any of these apply.

Some people need extra care with this medicine, or need their doctor to review them while taking it. A few conditions mean you must not take it at all, and those are listed in Who should not take O2 Tablet, just above.

• Children and teenagers under 18. Ofloxacin is not established as safe in people under 18, and children under 18 have a higher chance of bone, joint or tendon problems with it. Paediatric forms of this combination must be given only on a doctor's prescription.
• Older adults. The risk of tendon problems is higher, especially over 60 and especially if you also take steroid medicines. There is also a rare risk of a tear in the aorta, which is higher in older people and in people who already have an aortic aneurysm.
• Kidney problems. Ofloxacin is cleared by the kidneys. If your kidney function is reduced, your doctor may need to adjust your dose.
• Liver problems. Tell your doctor about any liver disease, so your dose can be checked.
• Diabetes. Blood sugar can rise or fall when fluoroquinolones are taken with insulin or with tablets such as glibenclamide. Monitor your blood sugar as your doctor advises.
• Epilepsy or other brain conditions. These medicines can increase the risk of seizures. Tell your doctor about any seizure history, or about severe headaches or confusion while taking them.
• Heart rhythm problems. Tell your doctor if you have a known long QT interval or low potassium, or if you take medicines that affect heart rhythm.
• Aortic aneurysm. If you have a known aortic aneurysm, or a family history of one, tell your doctor before taking it.
• Sunlight. Quinolones can make your skin unusually sensitive to sun and UV light. Avoid long sun exposure and tanning beds during treatment, and use protective clothing.
• Breastfeeding. Ornidazole passes into breast milk. Information on its effects on a breastfed baby is limited, and there is a concern about infant exposure to nitroimidazole medicines over long courses. Ask your doctor before breastfeeding while you take it. Where breastfeeding is interrupted around a dose, expert advice is to wait about three days before feeding again, which lets the amount in milk fall to a negligible level. If you are expressing, ask your doctor whether to discard that milk.
• Driving and machinery. Ofloxacin can make you feel dizzy or light-headed. Do not drive or operate machinery until you know how it affects you.
• Alcohol. See the interactions section below.

Some medicines do not mix well with O2 Tablet, and some change how well it is absorbed.

• Antacids, sucralfate, iron, calcium and zinc supplements, and buffered didanosine tablets. These reduce the absorption of ofloxacin. Leave at least a two-hour gap before and after them.
• Painkillers of the NSAID type, such as ibuprofen, may increase the chance of nervous system effects and seizures when taken with ofloxacin.
• Theophylline. Levels of theophylline in the blood can rise, so your doctor may need to check them.
• Warfarin and other blood thinners. The effect of the blood thinner can increase. Your doctor may arrange clotting tests.
• Diabetes medicines, including insulin. Blood sugar can drop, and in serious cases this has led to coma. Careful monitoring is needed.
• Other antibiotics or antiprotozoal medicines. Taking a second medicine that works against the same organisms is usually unnecessary and adds to resistance.
• Alcohol. Available evidence suggests that alcohol does not reduce how well fluoroquinolones work. However, a disulfiram-like reaction — flushing, nausea, vomiting, a fast heartbeat and a fall in blood pressure — has been reported with nitroimidazole antibiotics of the same group as ornidazole, and how often this happens is not clear. It is sensible to avoid alcohol while you are taking this medicine, and to ask your doctor what applies in your case.

Tell your doctor and pharmacist about every medicine, supplement and herbal product you take, including anything you buy without a prescription.

If you miss a dose, take it as soon as you remember — unless it is nearly time for your next dose. In that case, skip the missed dose and carry on with your usual schedule. Never take two doses together to make up for a missed one.

If you take too much, this needs medical attention even if you feel well at first. Go to the nearest hospital emergency department and take the strip or pack with you so the doctors know exactly what you have taken.

For a medical emergency in India, call 112. For an ambulance, call 108.

• Store the tablets at room temperature, 20 °C to 25 °C, in the container they came in, away from light and moisture - not in the bathroom or near a window.
• Keep them out of the reach of children.
• Check the expiry date and do not use the tablets after it.
• Do not keep leftover tablets "for next time". Return them to a pharmacy.
• Keep a note of when you started and finished the course, in case your doctor asks.