What I'm about to say will seem like medical heresy. But at this time of year, when the effects of seasonal indulgence lead many normally sensible people to begin planning over-ambitious New Year's resolutions, it's a message that is vital to get across before it's too late: exercise can be bad for you.

Of course, as a doctor, banging on about the benefits of exercise is normally right up there at the top of my daily to-do lists, along with telling people to quit their vices and refusing to give out antibiotics for colds.

But from experience, I know that while my current surgeries are full of people who want to be rid of their coughs and sore throats in time for Christmas, by the second week in January they will become a repository for those who have done themselves a mischief by attempting too much too soon in an effort to burn off their festive calories.

Exercise really can have some adverse effects. The most dramatic of these came during this year's Great North Run, when four seemingly healthy young runners died suddenly from previously undiagnosed cardiac problems. Thankfully, only one in 200,000 athletes are thought to be at risk of this. But there are a number of disabling conditions that are much more common.

The first is Achilles tendonitis (also known as tendinopathy), which is a common injury in people who go mad when they start running to increase their fitness. The Achilles tendon joins the muscles of the calf to the heel and, if overstretched, can become inflamed and painful.

These symptoms can usually be settled with a combination of anti-inflammatory drugs, rest (so you don't make it worse), ice (for about 20 minutes), compression with a bandage and elevation (both of which reduce swelling). The worst case scenario is rupture of the tendon. This requires surgical repair, and can put you in plaster for around six weeks - bringing any sort of exercise regime to a painful and abrupt end.

Plantar fasciitis is another condition that can affect joggers and walkers. The plantar fascia is a tough band of tissue running from the heel to the middle of each foot. If you do a lot of exercise when you're not used to it, this fascia becomes inflamed, leading to pain around the heel every time you walk or run. While rest and painkillers can help, many people need a steroid injection into the fascia to relieve the symptoms. And although more than one of my patients has said this hurts more than the condition itself, it is worth it in the end.

If you are thinking of taking up golf, a racket sport, swimming or even joining a gym to push some weights after the festive season, then the condition to avoid is Shoulder Impingement Syndrome.

This is caused by any activity that involves repeated movements of the arm over your head and leads to inflammation of the so-called rotator cuff muscles around the shoulder joint. Once swollen, they can become trapped in the joint itself, causing pain and reducing the shoulder's range of movement. Rest and ice can help, but many people need physiotherapy or steroid injections to ease the pain.

Patellofemoral dysfunction causes pain behind the kneecap and is another result of overzealous exercise such as running. It is typified by a constant dull ache that gets worse when going down the stairs or after sitting down for a long time. Rest, anti-inflammatories and physiotherapy are again the mainstays of treatment.

Despite all of these potential problems, the benefits of exercise - weight loss, lower blood pressure, stronger bones, better mental health - far outweigh the risks. And providing you are sensible and follow a few simple tips, your resolutions will have the desired effects of increased fitness and decreased waistline without resulting in a visit to the GP.

Here are some tips for sensible exercise:

· Exercise regimes should be started gradually by initially only running short distances or lifting light weights and then building up slowly to avoid overuse injuries.

· Ensure you have the right equipment and footwear, as blisters will stop you before you have really started.

· Warm up thoroughly with stretching exercises before each session. These can prevent muscles being pulled and inflamed joints and tendons.

· Drink plenty of fluids to avoid symptoms of dehydration.

· Eat a well-balanced diet - exercise uses a lot of energy and if you starve yourself beforehand, you risk passing out.

· Take some rest days to allow your muscles to recover.

· Make it fun.