Commonly used over-the-counter painkillers such as paracetamol, aspirin and ibuprofen will usually be strong enough to alleviate common aches and pains. But if you’re suffering from acute pain from dental work, minor surgery or migraine headaches, you may need something stronger.

So, how do you choose what’s best for you? And what are the side effects?

Pharmacy-only painkillers are usually rendered more effective by adding the opioid drug codeine. Adding codeine to paracetamol creates Panadeine from Panadol (or any other brand of paracetamol) or Nurofen Plus from ibuprofen alone.

Codeine is a naturally occurring opioid with a very long track record of relatively safe use. It constitutes around 3% of the alkaloids found in opium juice, but is synthetically derived for medical use.

Codeine has relatively poor analgesic ability by itself. Most of the painkilling effect of codeine is produced when metabolised by the liver. An average person will produce around 1mg of morphine from the 10 to 15mg of codeine in many of these over-the-counter analgesics.

But there is major variability in our ability to metabolise codeine. As many as 25% of the people in the community are unable to produce morphine from codeine and therefore will get very little pain relief. But they will endure the same side effects.

A much smaller percentage will be very active metabolisers who will produce a far higher percentage of morphine from the same dose.

In practice, adding codeine to paracetamol or ibuprofen does increase the overall effectiveness and is fine for occasional use for acute pain as long as you know you’re not one of those unlucky ones who doesn’t convert the codeine into morphine.

But codeine is too unreliable and modest in its effectiveness for use as a long-term treatment for persistent types of pain.

The side effects of codeine (apart from pain relief) can be quite significant. It is very constipating and can cause drowsiness, itchiness, nausea and a dry mouth. I encourage my students and registrars to think of codeine as a “constipating cough suppressant that gives some people pain relief as a side effect”.

In high doses, codeine can suppress the user’s breathing and cause drops in blood pressure on standing or sitting too quickly. In these respects, it’s typical of the opioid class.

Fatal overdoses involving codeine regularly occur, though in the case of compound analgesics, the paracetamol and ibuprofen may be more immediately damaging to the internal organs.

Adding an opioid to a formulation also leads to concerns about compulsive use and addiction. The tendency of all opioids to reinforce their own use by activating the dopamine reward system is perhaps the main reason why many people keep taking them when it’s clear that they’re having little effect on the pain they are supposed to be treating.

Dose increases due to tolerance are frequently an issue with drugs containing codeine, especially when treating chronic migraines and back pain. Your GP has a number of questionnaires you can take if you’re concerned about your risk of becoming addicted to codeine.

You should avoid over-the-counter codeine altogether if you are pregnant or breast-feeding. Due to their immature livers, children have much more erratic metabolism of codeine, and can accumulate potentially fatal levels of morphine from “average” doses if they are fast converters.

As codeine is excreted in the breast milk, it can cause side effects in the baby, and the same problems apply as for children. Always discuss the use of strong analgesia for children or while pregnant or lactating with your doctors.

Another ingredient added to some formulations is doxylamine succinate. Doxylamine is a first-generation antihistamine, and it is a very sedating one at that. Pharmacists and GPs sometimes use obsolete terminology to describe it as a “calmative” or “relaxant”.

The major brand of doxylamine-containing formulation is Mersyndol, which contains paracetamol and codeine as well. It’s often sold as a treatment for severe headaches. But this is the only paper I can find on its use in severe headaches. It’s a small study of migraine sufferers from 1976 – not exactly compelling evidence to support its current widespread use.

If you’re taking Mersyndol (or another drug containing doxylamine) more often than once a week on a regular basis, you should see a neurologist or other headache specialist to get a diagnosis as early as possible. Delayed diagnosis means your headaches can become much more difficult to control once the headache type is established and more specific treatments given.

Short-acting opioids or doxylamine are never recommended long-term treatment for any type of chronic headache.

Tips for choosing stronger painkillers: