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Confounding might have accounted for results

  1. Anne Szarewski, senior clinical research fellow in gynaecological oncology,
  2. Henry C Kitchener, professor
  1. Department of Mathematics, Statistics and Epidemiology, Imperial Cancer Research Fund, London WC2A 3PX
  2. Department of Obstetrics and Gynaecology and Reproductive Health Care, St Mary's Hospital, Manchester M13 0JH
  3. 148 Upper Clapton Road, London E5 9JZ
  4. Department of Internal Medicine, Altoona Hospital, Altoona, PA 16601-4899, USA
  5. 20 Coombe Ridings, Kingston-upon-Thames, Surrey KT2 7JU
  6. University of St Andrews, St Andrews KY16 9TS
  7. Royal London Hospital, London E1 1BB
  8. Farfield House, Keithley BD21 3AA
  9. PO Box 3AP, London W1A 3AP
  10. Department of Immunology, University College London Hospitals, London W1N 8AA
  11. Upper Weston, Llangunllo, Knighton LD7 1SL

    EDITOR—Beral et al have reported an increased relative risk of death from cervical cancer among women using oral contraception.1 Their study controlled for smoking but not for the other principal risk factors for cervical cancer, which include infection with human papillomavirus of a high risk type, sexual behaviour, and inadequate cervical screening. There is therefore a considerable risk of confounding, which could account for the results.

    The authors do not give patterns of use of contraception by never users and non-users of oral contraceptives; it would be interesting to know what proportion of this comparison group was using barrier methods Such women are at reduced risk of cervical cancer,2 and therefore a comparison that includes a large proportion of women using barrier methods will seem, artificially, to increase any risk in pill users.

    It should also be noted that durations of use of less than 10 years are not associated with a significant increase in risk in this study This apparent duration of use effect may well be related to the risk of acquiring an infection with an oncogenic human papillomavirus.

    References

    1. 1.↵
      1. Beral V,
      2. Hermon C,
      3. Kay C,
      4. Hannaford P,
      5. Darley S,
      6. Gillian R
      . Mortality associated with oral contraceptive use: 25 year follow up of cohort of 46 000 women from Royal College of General Practitioners' oral contraceptive study. BMJ 1999; 318: 96–100. (9 January.)
      OpenUrlFREE Full Text
    2. 2.↵
      1. Schottenfeld D,
      2. Fraumeni JF
      1. Schiffman MH,
      2. Brinton LA,
      3. Devesa SS,
      4. Fraumeni JF
      . Cervical cancer. In: Schottenfeld D, Fraumeni JF, eds. Cancer epidemiology and prevention. Oxford: Oxford University Press, 1996.

    Relative risk of liver cancer remains high

    1. Joseph Horowitz, general practitioner
    1. Department of Mathematics, Statistics and Epidemiology, Imperial Cancer Research Fund, London WC2A 3PX
    2. Department of Obstetrics and Gynaecology and Reproductive Health Care, St Mary's Hospital, Manchester M13 0JH
    3. 148 Upper Clapton Road, London E5 9JZ
    4. Department of Internal Medicine, Altoona Hospital, Altoona, PA 16601-4899, USA
    5. 20 Coombe Ridings, Kingston-upon-Thames, Surrey KT2 7JU
    6. University of St Andrews, St Andrews KY16 9TS
    7. Royal London Hospital, London E1 1BB
    8. Farfield House, Keithley BD21 3AA
    9. PO Box 3AP, London W1A 3AP
    10. Department of Immunology, University College London Hospitals, London W1N 8AA
    11. Upper Weston, Llangunllo, Knighton LD7 1SL

      EDITOR—Beral et al state that, for ever users of oral contraceptives compared with never users, both the overall relative risk of death and the relative risk of death from all cancers are 1.0.1 They discuss the fact that the relative risk of cervical cancer is 1.7 but ignore …

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