In addition to the other excellent answers, which cover that some doctors do indeed prescribe the smoking of MMJ, some would suggest that the reason for this is the differences in the psychoactive effects rather than the medical benefits involved in the various methods of ingestion.
According to United Patient's Group, a MMJ information and advocacy site says of eating cannabis-infused foods (emphasis mine):
The effects of consumed cannabis is much different compared to smoking
or vaporizing. Edibles are slower to kick in, slow to wear off and
usually give more of a "body" versus "head" high, an effect described
as "heavier" or "deeper" than if smoked or inhaled. This can be
particularly beneficial for those with chronic severe body pain.
A word of caution to those choosing to medicate with edible marijuana
- unlike with smoking and vaporizing, it is much easier to over-consume, and therefore over-medicate with ingestion. Because it
can take longer to feel the effect and/or because the edibles taste
good, patients are warned to [...] be extra careful in consumption so
as not to exceed recommended dosage.
So it could be that those wishing to avoid being "stoned" would prefer a method that make estimating dosages simpler at home. This could, in part, be due to the lack of major pharmaceutical development; each dispensary might have products of very different dosages, depending on preparation techniques.
It also point out other, less unhealthy alternatives to smoking:
Another alternative route of administration for medical marijuana free
from any psychoactive reactions is to use a topical made from cannabis
and apply it directly onto the skin. Marijuana topicals, such as
balms, salves, lotions, sprays and ointments made from cannabis oils,
can be very effective analgesics and anti-inflammatories.
Use of a marijuana vaporizer is the most recommended method as an
alternative to smoking. [...]
How the Medical Marijuana Vaporizer Works: A vaporizer heats the
cannabis plant slowly causing the active ingredients to evaporate into
a vapor without reaching the point of combustion, thereby releasing a
much lower proportion of other harmful components that come from
So it would appear that many methods of ingestion are recommended. MarijuanaGrowersHQ.com does point out that smoking probably the most harmful way of ingesting cannabinoids, though it is difficult to verify this from scinetific studies:
Unfortunately inhaling smoke is not the most healthy method of
consuming cannabis. While there is still no conclusive correlation
between the effects of smoking cannabis and an increased incidence of
lung cancer, this is likely due, in part, to the difficulty in finding
subjects with a history of long term cannabis use but no history of
No source is provided for this claim, however.
So it would seem that while some doctors do prescribe the smoking of medical cannabis, others recommend another method of ingestion. Those who do recommend must believe convenience and dosing consistency outweigh other concerns.