Thank you for your inquiry.
There is a fairly rich body of literature on allergic reactions to marijuana which give us some insight as to potential cross-reactivity with other pollens and foods. However, there is no definitive literature of which I am aware that answers your question specifically regarding weeds.
Nonetheless, from this body of literature (references and abstracts are copied below), it is clear that marijuana can be an allergen not only by inhalation but also by ingestion.
The ingestion of hemp seed (Cannabis sativa) has been reported to cause anaphylaxis as you can see from the 2003 Journal of Allergy and Clinical Immunology abstract copied below.
In addition, the inhalation of marijuana pollen has been known to produce respiratory allergic symptoms (see Stokes, et al., below). It is interesting to note in this article that patients who have respiratory allergy to marijuana pollen also had positive skin tests to weeds.
An extensive classification of all allergens in marijuana has not, to my knowledge, been performed. However, we do know that there is a lipid transfer allergen which shares cross-reactivity with a number of different foods (see Metz-Favre, et al., below).
Also there has been some suggestion that marijuana seed shares cross-reactivity with other seeds.
Thus, what we can conclude from the available literature is that marijuana is capable of producing allergic reactions both in the respiratory tract via its pollination and by ingestion, causing anaphylaxis. We also know that at least one allergen in marijuana can be found in other foods and pollens. Therefore, although I was not able to find any specific literature regarding cross-reactivity between marijuana and weeds per se, there is evidently a risk of such cross-reactivity.
One thing you might consider, based on procedures noted in the references below, is to perform a skin test to marijuana prior to its administration.
Thank you again for your inquiry and we hope this response is helpful to you.
Allergic Skin Test Reactivity to Marijuana in the Southwest
Geraldine L. Freeman, MD: West J Med. 1983 June; 138(6): 829–831.
In a general allergy consultation practice in Arizona and western New Mexico, 129 patients were tested for immediate hypersensitivity skin test reactivity to marijuana pollen and tobacco leaf, as well as to a battery of other antigens. In all, 90 patients were diagnosed as allergic (atopic) and, of these, 63 (70 percent) were found to be skin test reactive to marijuana pollen and 18 (20 percent) to tobacco leaf. The incidence of skin test reactivity to marijuana was not significantly different for persons living at low, middle or high elevations throughout the Southwest. Marijuana sensitivity occurred in patients who were, in general, also sensitive to a variety of other airborne plant pollens. There was no close correlation, however, between sensitivity to marijuana pollen and sensitivity to pollens from elm, mulberry, hop and stinging nettle, which are botanically related to marijuana. The data suggest that marijuana pollen may be a relatively common airborne pollen pollutant in the Southwest, allergic persons being sensitized through inhalation. If this is confirmed by further studies, then clinical investigation of marijuana hyposensitization (immunotherapy) may be warranted. This is in contrast to tobacco allergy for which simple avoidance is recommended.
Annals of Allergy, Asthma & Immunology, Volume 85, Issue 3 , Pages 238-240, September 2000. Stokes, et al
Cannabis (hemp) positive skin tests and respiratory symptoms
Background: We have noted several patients who had rhinitis and/or asthma symptoms when exposed to Cannabis plants in the summer months. Cannabis plants are common in the Midwest.
Objectives: To examine whether Cannabis might be a clinically important allergen, we determined Cannabis pollination patterns in the Omaha area for 5 years, the prevalence of skin test positivity, and the association with respiratory symptoms.
Methods: Airborne Cannabis (and other weed) pollens were collected using a Rotorod air impactor, and pollen counts were done using a standardized protocol.
Results: Measurable Cannabis pollen count was not recorded until the last 2 weeks of July. Peak pollination typically occurred during mid- to late-August, and comprised up to 36% of the total pollen counts. Cannabis pollen was not observed after mid-September. To determine the prevalence of skin test positivity, we added Cannabis to the multi-test routine skin test battery. Seventy-eight of 127 patients tested (61%) were skin test positive. Thirty of the 78 patients were randomly selected to determine if they had allergic rhinitis and/or asthma symptoms during the Cannabis pollination period. By history, 22 (73%) claimed respiratory symptoms in the July through September period. All 22 of these subjects were also skin test positive to weeds pollinating during the same period as Cannabis (ragweed, pigweed, cocklebur, Russian thistle, marsh elder, or kochia).
Conclusions: The strong association between skin test reactivity, respiratory symptoms, and pollination period suggests that Cannabis could be a clinically important aeroallergen for certain patients and should be further studied.
The Journal of Allergy and Clinical Immunology, Volume 127, Issue 2, Supplement , Page AB178, February 2011
Rationale: Marijuana can be used through inhalation or through oral ingestion. Allergy to marijuana is considered rare, but a few reports of allergies to marijuana have been documented and lipid transfer protein was recently identified as an allergen, in one case. Here we report seven patients that presented with allergic symptoms associated with marijuana exposure.
Methods: A detailed history of exposure to marijuana and symptoms was taken from each patient. Skin prick tests (SPT) were used to confirm allergy to marijuana, using the bud or flower of the marijuana plant.
Results: All seven patients had large positive SPT with wheals larger than 5 mm and surrounding flare, confirming their allergy to marijuana. Six patients presented with inhalation symptoms with exposure to marijuana. Inhalant symptoms included rhinitis and conjunctivitis in five, periorbital angioedema in three, sinusitis in two, wheezing in two, and swelling of the throat sensation in one case. Six of the patients also presented with contact symptoms which included five occurrences of urticaria, one of periorbital angioedema, and one of dermatitis. One patient presented with anaphylaxis symptoms which included anxiety, tightness of chest, wheezing, GI cramping and vomiting with ingestion of a marijuana tea.
Conclusion: It appears that marijuana may be a much more common allergen than previously thought. Allergic reactions to marijuana may become more of an issue in the future given the increasing social use of marijuana, as well as its expanding medical use. It is important that marijuana exposure is addressed when assessing a patient's exposure history.
The Journal of Allergy and Clinical Immunology, Volume 112, Issue 1 , Pages 216-217, July 2003. Anaphylaxis to ingestion of hempseed (Cannabis sativa).
Metz-Favre C, et al. Molecular Allergology in Practice: Unusual Case of LTP allergy. European Annals of Allergy and Clinical Immunology 2011; 43(6):193-195.
Phil Lieberman, M.D.