Dr MacAllister

In the novel Peril at End House, Dr MacAllister is the uncle of Commander Challenger. He has a practice on Harley Street. Japp described him as "a woman's doctor". By that he did not mean a gynaecologist. He thought of Macallister as "one of these nerve doctors—tell you to sleep in purple walls and orange ceiling—talk to you about your libido, whatever that is—tell you to let it rip." Japp regarded him as a quack but the doctor did manage to attract a large number of women patients. MacAllister went abroad frequently and appeared to be doing some kind of medical work in Paris.

Poirot later revealed that MacAllister had been using his clinic as a centre for the distribution of drugs such as cocaine. Commander Challenger was part of the operation. He would go to the clinic to collect the drugs and then distribute them to his contacts concealed in wrist watches.

Agatha Christie's Poirot
In ITV's 1990 TV film adaptation, Dr MacAllister is mentioned although he does not appear on screen. In this adaptation, the facts about the doctor are not given by Japp. Poirot sends Miss Lemon undercover as a patient to his clinic and she came back with a detailed report. Miss Lemon also described the docter as a "woman's doctor". To her it was a general term for "neurotic". She observed that there was something unpleasant about the clinic although it was highly respectable and included among the patients well-known personalities such as "Lady Lowestoft" and "Mrs Bindhoff". None of these patients seemed to be with the doctor for more than five minutes. Miss Lemon also found out that Commander Challenger came to the clinic at least once every then days. Like in the original novel, Dr MacAllister also ran a nursing home in Paris.