Background: Asthma, allergic conditions, and COPD overlap, but the effect of them and their combinations on disease severity, need for drugs, use of healthcare, and costs is poorly known. Objective: To study how different allergic diseases co-occur in asthma and allergy patients and evaluate the use of medication well as drug and healthcare costs. Methods: Nationwide Allergy Barometer Survey was carried out in the Finnish pharmacies during 1 week in September 2016. Altogether, 956 patients (5–75 years) who purchased asthma or allergy drugs with prescription participated in 351 pharmacies. Results: Of the participants, 78% reported physician-diagnosed asthma, 57% allergic rhinitis, 24% atopic eczema, 21% food allergy, 20% allergic conjunctivitis, 8% anaphylaxis, and 8% COPD. One-third of the patients had at least three conditions, and multimorbidity was common across all age groups. Disease severity increased with the number of coexisting conditions, and asthma severity also with age. Patients with asthma alone used on average 3.8 drugs with the annual costs of EUR 661. This increased to 4.9 drugs and EUR 847 in asthmatics with multimorbidity. For all participants, costs of drugs and healthcare services together during the preceding year were on average EUR 1,214, of which 56% were drug costs. The costs doubled to EUR 2,714 in 65 subjects (7%) who had both asthma and COPD. Conclusions: In asthma and allergy, multimorbidity and polypharmacy are major concerns. Disease severity, drug use, and costs increased with multimorbid conditions. To reduce the burden, allergy management should be better integrated and more comprehensive.