Every week, it seems, a scientific study appears disproving what last week's study showed. Recently, there was a classic medical volte-face: aspirin, which has been prescribed to millions of people over the decades as a protective measure against heart disease, may have more drawbacks than benefits, according to a review in the Drug and Therapeutics Bulletin.

Although a daily aspirin helps prevent a second heart attack or stroke in people who have already had one, in healthy people any protection against cardiovascular disease may be outweighed by an increased risk of internal bleeding, researchers say. Bleeding is a well-known side effect of aspirin and similar drugs that act as irritants to the stomach lining. After years of headlines about the benefits of aspirin, the recent one read: "Aspirin is bad for you".

In the past month, we have also learnt that a father's presence at childbirth is bad for the mother, that drinking three cups of coffee a day protects against liver disease (for people with hepatitis C) and that consuming alcohol cuts a woman's chances of conceiving by IVF. Yet fathers have been encouraged to attend childbirth for decades, coffee has been implicated in umpteen health scares, and alcohol is known to be good for the heart.

A is for alcohol
Good: Moderate drinking is good for the heart, though the effect is chiefly seen in middle-aged men - two or three alcoholic drinks a day cuts the risk of heart attack by at least 30 per cent.

Bad: Drinking to excess, liver disease, and alcohol dependency are all rising. Other effects are less obvious: one drink a day increases a woman's risk of all types of cancer by six per cent. Harvard Medical School scientists earlier this month presented findings showing couples having IVF who drank one bottle of wine a week cut their chances of a live birth by a quarter.

B is for beta blockers
Good: They are among the most widely prescribed drugs for preventing heart attacks in people with high blood pressure but without existing heart disease.

Bad: For patients undergoing surgery - millions of whom have been given them to reduce the risk of heart attack following an operation - they can be health-adverse. Last year, an international study concluded that the practice had caused 800,000 deaths worldwide due to an increased risk of a stroke.

C is for coffee
Good: Caffeine is the world's most widely used stimulant drug and coffee is the form in which millions of people prefer to take it. It improves short-term memory, boosts muscle power, raises alertness - and tastes delicious.

Bad: Coffee is linked with an increased risk of heart disease, arthritis, stillbirth and raised blood pressure.

D is for dieting
Good: A good idea fo getting into that wedding dress, preparing for the beach, dealing with Christmas excess. But much harder to sustain.

Bad: Diets can ruin the lives of anorexics, bulimics and all other diet-obsessives. Yo-yo dieting increases the risk of heart disease, kidney cancer and osteoporosis.

E is for exercise
Good: Great for the heart, the lungs, the muscles, circulation - you name it. Helps prevents heart disease, cancer and extends life.

Bad: Beware of joint problems if you run a lot (more than 32km a week) without proper shoes and on hard surfaces. Heart attacks do occur (Jim Fixx, the father of jogging, died of one), though rarely, and mostly in those taking up exercise for the first time.

F is for folic acid
Good: It helps to prevent spina bifida in unborn babies. Pregnant women in New Zealand are advised to take 400 micrograms of folic acid daily.

Bad: Elderly people with pernicious anaemia, a deficiency of vitamin B12, should not take folic acid supplements. The folic acid may mask the deficiency which can have neurological effects if undiagnosed. Fears folic acid increases the risk of bowel cancer have been discounted by Britain's Food Standards Agency.

G is for genetic testing
Good: Those unlucky enough to have inherited genes that increase their risk of disease - such as BRCA1, which raises the lifetime risk of breast cancer to 80 per cent - can benefit from such testing. They can then have annual mammograms to treat cancer early or undergo preventive mastectomy to avoid cancer developing.

Bad: For people with genes for conditions such as Huntingdon's disease or Alzheimer's that are untreatable, learning they are at risk blights their lives without conferring benefit.

H is for holidays
Good: For everyone - relaxation, fun, unwinding, time with the family, travel, change of routine.

Bad: For - this may come as a surprise - mental health. The World Health Organisation says mental problems are among the leading causes of ill health for travellers and "psychiatric emergency" is one of the most common reasons for evacuation by air ambulance.

I is for infection
Good: Infection can be good for young children, who need exposure to viruses and bacteria in early life to develop their immune systems. Those who miss out on such exposure are more likely to develop allergies, such as asthma, later on.

Bad: For just about everybody else.

J is for juice
Good: For those who don't like fruit and prefer to have it squeezed for them, juice is good. But it only counts as one portion towards your five-a-day, no matter how much you drink.

Bad: Consider your teeth - the acid in orange juice rots the enamel - and kids who drink too much lose their appetite for solid food, a condition doctors call "juice-drinking syndrome".

K is for kissing
Good: The way to welcome, congratulate, comfort, seduce, foster intimacy and farewell.

Bad: There is a risk of catching the "kissing disease" - glandular fever - which is spread in saliva. Also strep throat, cold sores, colds and flu. Hepatitis B can be transmitted by kissing.

L is for lasers
Good: Lasers can zap unsightly blemishes on the skin, cauterise blood vessels, correct eyesight, clean arteries and drill teeth.

Bad: There is a risk to patients going for eye laser treatment of ending up with blurred vision and dry eyes. In the US, the Food and Drug Administration is investigating the level of side effects from the treatment.

M is for milk
Good: For children born a generation ago, who needed the calcium for developing bones and keeping blood pressure down, milk was the answer.

Bad: Adults and children of today eat a much richer diet. Thickening of the arteries leading to heart disease begins in childhood and high-fat foods - like whole milk, butter and cheese - are key culprits.

N is for nuts
Good: They're good for lowering heart disease risk, according to Harvard School of Public Health. Also vegetarians can benefit as nuts are an important source of protein.

Bad: Steer clear if you have a nut allergy - reactions can range from mild (rashes and spots) to severe (death).

O is for oily fish
Good: Thanks to the Omega 3 fatty acids it contains, fish can protect against heart disease in adults, and boost brain function and IQ in children, and in the developing foetus.

Bad: Mercury, a toxic chemical which is absorbed by oily fish, can affect IQ and increase the risk of heart disease, thus countering the beneficial effects of the Omega 3s.

P is for painkillers
Good: Like it says on the packet, they will kill pain - whether you use aspirin, paracetamol, ibuprofen or stronger drugs.

Bad: For the stomach. Many people end up in hospital each year because of internal bleeding and ulcers caused by painkillers.

Q is for queues
Good: For ensuring the neediest health patients get seen first. Patients may queue for hours to be seen in A&E or for weeks for a hospital appointment.

Bad: For those who have waited too long. Here, private hospitals are to be given a greater role in carrying out taxpayer-funded elective surgery under a Government plan to treat more patients and cut hospital waiting lists.

R is for roughage (fibre)
Good: Fibre helps the bowels, digestion and may prevent cancer of the gut. It can also reduce the risk of heart disease.

Bad: Flatulence, abdominal discomfort and diarrhoea can be side effects.

S is for sunshine
Good: For making vitamin D, which is essential for healthy bones and to protect against a range of diseases including cancer, heart disease and multiple sclerosis.

Bad: Harsh sun can cause sunburn which can trigger melanoma, the most lethal form of skin cancer. This led to doctors telling people to cover up and use sunscreen. But the message has been modified because of the importance of vitamin D. A little sun is good for you but too much could be dangerous.

T is for tea
Good: Black tea reduces heart disease. It also reduces stress, boosts mental alertness and may boost the immune system and help prevent diabetes.

Bad: Adding milk counteracts its effects. Sugar makes it worse.

U is for underwear
Good: For fashion, support, sex, hygiene.

Bad: It can affect fertility in men when worn too tight. Testicles need to be below body temperature to produce sperm.

V is for vitamins
Good: Good for maintaining health, particularly in the sick or elderly, who may be vitamin deficient because they are not eating properly or cannot absorb vitamins from their diet.

Bad: Supplements are seen as a modern panacea but they do little for most people other than to create expensive urine. They are thought to be unnecessary if you are healthy and eating a balanced diet.

W is for water
Good: We cannot live without it, but how much is enough? The common advice is that humans need 2.5 litres daily - but that includes liquid from all sources including food.

Bad: Too much can result in water intoxication, hyponatraemia (low salt levels) and even death.

X is for X-rays
Good: Good for diagnosing broken bones, decayed teeth and disease.

Bad: X-rays are the largest man-made source of radiation to which we are exposed. Research suggests the radiation emitted by X-rays causes six of 1000 cases of cancer a year.

Y is for yoghurt
Good: Probiotic yoghurts and drinks are sold with the promise they can improve digestive health.

Bad: A review by the European Food Standards Agency last month rejected all 180 health claims made for probiotic ingredients.

Z is for zzzz
Good: A few hours of quality sleep is better than eight hours thrashing about. Most adults manage with seven to nine.

Bad: For shift workers, a body clock out of phase with the environment is thought to be harmful, affecting production of the hormone melatonin.