Pharmacy and supermarket shelves are stacked with cold and flu remedies promising to fix your winter ills. But despite the marketing hype, there's not much evidence that many of these products actually work. At best, they may provide temporary relief from your symptoms. How do you know if they're worth paying for?

Over-the-counter treatments

Anti-bacterial hype

Colds and flu are caused by viruses, not bacteria. But there's a rash of anti-bacterial products on shop shelves.

Strepsils Orange lozenges boast "double antibacterial action" to soothe sore throats. Sucking a lozenge will increase the flow of saliva and soothe your throat, although a barley sugar or spoon of honey will do something similar.

These lozenges – or any other anti-bacterial lozenge – can’t do anything to cure a cold. Reckitt Benckiser, the manufacturer of Strepsils, told us its lozenges have "never been marketed for cold and flu". Yet pharmacies we visited in central Wellington all recommended Strepsils as a treatment option for a sore throat associated with a cold (some suggested a lemon and honey drink would work just as well).

Anti-bacterial lozenges may be helpful if you have an underlying bacterial infection with your cold. Medsafe says the active ingredients in Strepsils Orange (amylmetacresol and dichlorobenzyl alcohol) are known to have antiseptic properties. But whether they'll speed your recover is another matter.

The Strepsils packet we bought states: "The efficacy of an antibacterial agent in lozenges in reducing the severity or duration of throat infections has not been clinically established."

Hand washes
We don't think there's much point spending money on products like anti-bacterial hand washes if you've got a cold or flu. According to the US Centers for Disease Control and Prevention (CDC), it hasn’t been proved that these hand washes are any better at stopping the spread of infection than similar products without anti-bacterial chemicals.

There's also a risk they could do harm. The CDC says that laboratory tests have shown a link between anti-bacterial chemicals in cleaning products and bacterial resistance. But further research is needed to determine whether these products may contribute to bacterial resistance outside of the lab.

Repackaged painkillers

Some cold and flu remedies are little more than common painkillers with a higher price tag.

The Lemsip Cold and Flu Hot Drink we bought was essentially paracetamol in a sachet with added sugars and sodium. Each Lemsip sachet contained 500mg of paracetamol, the same amount you'd find in a common paracetamol product like Panadol. But you can end up paying nearly six times more for the Lemsip.

Analgesics such as paracetamol, aspirin and ibuprofen can relieve general aches and pains associated with colds and flu. In fact, they appear to be one of the few remedies that experts agree will relieve symptoms. If you already have them in your medicine cabinet, you don't need to buy more.

Cough medicines

Over-the-counter cough medicines may relieve symptoms for some people. Yet several studies have found these products work no better than a placebo. At least one study found that plain honey was better in reducing symptoms in children than remedies containing dextromethorphan (a common cough-suppressant).

A 2007 Cochrane Review (an analysis of drug trials and other medical research) concluded there's no good evidence either for or against the use of over-the-counter medications for acute coughs. Where they do provide relief, the effect may be from the added sugars: cough medicines often contain sweeteners such as glycerine, honey and syrup, which coat the throat and relieve the irritation that causes coughing.

Combination medicines

Many cold and flu remedies contain a combination of active ingredients – for example, a pain reliever together with a decongestant and an antihistamine. This king-hit approach isn't necessarily a good way to deal with your symptoms.

For a start, combination medicines can create dosage problems. Many remedies contain paracetamol – so if you take a couple of Panadol and a little later you take a cold remedy, you have to make sure the remedy doesn't contain paracetamol as well. Otherwise you could be doubling your daily intake.

Maree Jensen, academic director of the School of Pharmacy at Auckland University, says paracetamol is often taken in "hidden" forms in cold remedies at the same time as regular paracetamol. Because these cold remedies are readily available from dairies, supermarkets and petrol stations, people assume they're “safe”, she says. But taken too often or in large doses, paracetamol has potentially serious results and can cause liver damage.

Some of the active ingredients in combination remedies can also work against each other. Cough remedies can contain both the cough suppressant dextromethorphan and an expectorant called guaifenesin – one is intended to stop you coughing and the other to thin mucus and make you cough more. Combining these ingredients in one product doesn't seem to make much sense – even if the cough medicine worked.

What might work

According to the British Medical Association, antihistamines and decongestants have been shown to be "somewhat effective" in relieving symptoms but the benefit may be small.

Antihistamines help dry up mucus in the nasal passages, making your nose slightly less runny. They can also reduce sneezing. However, the use of antihistamines to treat cold and flu systems probably won't do anything to help your overall recovery. These ingredients can also have adverse side effects, including drowsiness, headaches, and dry mouth.

Some antihistamines such as doxylamine and triprolidine can interfere with other medications. You can't use them if you're taking certain anti-depressants. They're also unsafe for people with medical conditions such as glaucoma and prostate problems.

Decongestants work by constricting blood vessels in the nose, throat and sinuses. This reduces swelling and mucus formation and can help make breathing easier.

On the flipside, oral decongestants containing pseudoephedrine or phenylephrine make some people agitated – and they may not be suitable if you have heart disease, diabetes, hyperthyroidism, or high blood pressure. Sales of pseudoephedrine are controlled because of its use in illegal drugs (it's a key ingredient in "P" manufacture).

Using a decongestant nasal spray too often or for too long can also cause "rebound" congestion. This means your blocked nose will come back when you stop taking the product.

Pain and fever
Aspirin, paracetamol, ibuprofen Relieve pain and reduce fever. Aspirin and ibuprofen also reduce inflammation.
Runny nose
Antihistamines (eg brompheniramine, chlorphenamine, diphenhydramine, doxylamine, promethazine and triprolidine) May help dry up runny noses and reduce sneezing.
Blocked nose
Decongestants (eg pseudoephedrine and phenylephrine) May help unblock your nose by reducing blood supply and swelling.
Cough suppressants (eg dextromethorphan) or cough expectorants (eg guaifenesin) Suppressants are meant to stop you coughing; expectorants are meant to help you cough up mucus. But there's no strong evidence on whether over-the-counter cough medicines are effective.

Unsafe for under 6

Many cold and flu remedies aren't suitable for children. The risk of adverse effects prompted Medsafe to update its advice on these products in 2009. It says the following substances should not be given to children under the age of 6:

  • guaifenesin or ipecacuanha (found in cough expectorants)
  • dextromethorphan or pholcodine (found in cough suppressants)
  • pseudoephedrine or phenylephrine (found in decongestants)
  • brompheniramine, chlorphenamine, diphenhydramine, doxylamine, promethazine or triprolidine (found in antithistamines).

Restrictions introduced on 1 May 2011 mean preparations for children under 12 that contain dextromethorphan (cough suppressants) or phenylephrine (decongestants) can be sold only in pharmacies.

Natural remedies

Echinacea, vitamin C and zinc have been variously touted as a treatment for colds and flu.

  • According to a 2009 Cochrane Review, there's some evidence echinacea preparations derived from the species Echinacea purpurea might be effective for the early treatment of colds in adults (although the results aren't consistent). Evidence for the effectiveness of other echinacea preparations is weak.
  • There's not much support for vitamin C supplements. Regular intake of vitamin C may reduce the risk of catching a cold for people under physical stress – such as high performance athletes – but it won't ward off colds for most people.
  • Zinc appears to hold more promise. A 2011 Cochrane Review concluded that zinc was beneficial in reducing the duration and severity of the common cold in healthy people when it was taken within 24 hours of the onset of symptoms. But the review concluded there was insufficient evidence for making any firm recommendations about dosage.

Treatment without medicines

A cold will usually clear up on its own within 5 to 7 days. Flu usually lasts 1 to 2 weeks.

Mild symptoms can be relieved without buying over-the-counter remedies.

  • Bed rest is probably the best treatment – it’s likely to make you feel more comfortable (and staying home means fewer people will be exposed to your infection).
  • Drink plenty of fluids to counteract the dehydrating effect of fever.
  • Sucking sweets increases the flow of saliva and soothes your throat.
  • Hot liquids (drinks, syrups and soups) also soothe the throat.

If you decide it's time to try an over-the-counter product, follow the advice on the label. Some ingredients in cold remedies can be harmful in large doses or if taken over long periods. If your symptoms persist – or if they’re severe – talk to a health professional.